github provider: allow multiple teams

Add GitLab provider

Added SystemD example service file

Fix small typo in README.md.

fixed formatting

Since most people want to run this as a service, having some sort of an
example file is very useful to have.

use Github provider with GitHub enterprise

print the error stack trace

point to my fork in oauthproxy.go

addint pointers to ruta-goomba fork in mulitple files

change api endpoint

replace hard-coded github api endpoint with variables

resetting fall through github urls to point to github.com

fix malformed url

changes to enable use with enterprise github

Validate state param while redirecting.

Ignore spaces in authorized emails list

Google UI changes

Google changed to developer console UI, updated walkthrough to match new UI.

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