Add support for setting the basic auth password.

For tools that don't like empty passwords, this change allows
one to set a shared secret password for all users.

Check email validity on all requests rather than only on login.

Google - continually use refresh token

* New SessionState to consolidate email, access token and refresh token
* split ServeHttp into individual methods
* log on session renewal
* log on access token refresh
* refactor cookie encription/decription and session state serialization

Google - use offline access token

Cookie Refresh Improvements

* refresh now calculated as duration from cookie set

Release Version Two

* bump version to 2.0
* remove --cookie-https-only option
* add windows build to dist.sh
* rename --cookie-key to --cookie-name

Native SSL support

Page defaults to Google sign in

Github provider "Invalid Account"

disable email validation

This adds a "*" option to --email-domain to disable email validation, and this renames `--google-apps-domain` to `--email-domain` for clarity across providers

Fix validator_test hang on Solaris, Plan 9

On these platforms, the `done <- true` statement in during TearDown() was
hanging, since the `watcher_unsupported.go` version was never draining the
channel. Also took the opportunity to update the WatchForUpdates() signature
to not return bool anymore.

Fix unsupported WatchForUpdates and build tags

Closes #105.

The `go1.1` constraint has been updated to `!go1.3` per
https://golang.org/pkg/go/build/#hdr-Build_Constraints and
https://github.com/go-fsnotify/fsnotify. Also, the existing call signature for
`WatchForUpdates()` in `watcher_unsupported.go` was stale.

rename example file which is referenced in the README

Naming Is Hard - project name change

Add Github Provider

Provide graceful shutdown of file watcher in tests

TestValidatorOverwriteEmailListViaRenameAndReplace was deadlocking on
Windows because, on Windows, fsnotify.Watcher will continue to watch a
renamed file using its new name. On other systems, it appears the watch on
a file is removed after a rename.

The fix is to explicitly remove the watch to ensure the watch is resumed
under the original name.

These test failures from #93 inspired this change:
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474406
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474407

Both tests exhibited this pattern:
2015/05/13 22:10:54 validating: is xyzzy@example.com valid? false
2015/05/13 22:10:54 watching interrupted on event: "/tmp/test_auth_emails_300880185": CHMOD
2015/05/13 22:10:54 watching resumed for /tmp/test_auth_emails_300880185
2015/05/13 22:10:54 reloading after event: "/tmp/test_auth_emails_300880185": CHMOD
panic: test timed out after 1m0s

[snip]

goroutine 175 [chan send]:
github.com/bitly/google_auth_proxy.(*ValidatorTest).TearDown(0xc2080bc330)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:27 +0x43
github.com/bitly/google_auth_proxy.TestValidatorOverwriteEmailListViaRenameAndReplace(0xc2080f2480)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_watcher_test.go:103 +0x3b9

[snip]

goroutine 177 [chan send]:
github.com/bitly/google_auth_proxy.func·017()
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:34 +0x41

I realized that the spurious CHMOD events were causing calls to
`func() { updated <- true }` (from validator_test.go:34), which caused
the goroutine running the watcher to block. At the same time,
ValidatorTest.TearDown was blocked by trying to send into the `done` channel.
The solution was to create a flag that ensured only one value was ever sent
into the update channel.