Add LinkedIn provider

Make address/domain comparisons case-insensitive

Refactor pass_access_token changes from #80

Extracts buildCookieValue() and parseCookieValue() from OauthProxy.ServeHTTP()
and adds tests for both.

Moves the check from NewOauthProxy() to Options.Validate() and adds a test.

Redirect to / when /oauth2/sign_in accessed

Without this change, clicking the sign-in button on /oauth2/sign_in will
always redirect back to /oauth2/sign_in, essentially creating an infinite
loop.

Pass the access token to the upstream server

This is accomplished by encoding the access_token in the auth cookie and
unpacking it as the X-Forwarded-Access-Token header for upstream requests.

This reflects the apparent intent of TestNewReverseProxy(). Without this
change, the test will fail when run without an Internet connection.

Add myusa provider

Add ProviderName field; use in sign_in template

Extract providers package

Extract api package

This is the first step towards genericizing the google_auth_proxy to support
OAuth2 providers other than Google as discussed in #65. The `api` package will
enable multiple providers to use the same `api.Request()` implementation.

request path contains full URL

Cut a new release

Request Logging:

clear cookie fix

Rename flag to set secure (https) cookies

make redirect-uri optional