Add /auth endpoint to support Nginx's auth_request

Closes #152.

Go code style

Be consistent with Go capitalization styling and use a single way of
spelling this across the tree.

Be consistent with Go coding style for acroynyms.

Go coding style says that acronyms should be all lower or all upper. Fix
Url to URL.

Pass `ProxyPrefix` into the error template.

The default `error.html` uses `ProxyPrefix` but it isn't supplied in the context, causing it to error.

Add support for serving static files from a directory

The path should be provided as a file:// url with the full operating system path.
An alias to where the directory is available as can be specified by appending
a fragment (ie. "#/static/") at the end of the URL.

[RDY] google: Support restricting access to a specific group(s)

Allow passing the value of "approval_prompt" as a flag or option.

Update github.go

Google auth configuration screen flow has changed

By setting this to "force", certain providers, like Google,
will interject an additional prompt on every new session. With other values,
like "auto", this prompt is not forced upon the user.

provider github not work with scope read:org 

Fix spelling

*snicker*
*titter*
*giggle*

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