Add support for OAuth Web Flow

This adds support for developers who want to implement the OAuth web flow
with GitHub authentication. https://developer.github.com/v3/oauth/
Implement OAuth Web Flow

Provide methods to make it easy for developers to implement the
web flow.

https://developer.github.com/v3/oauth/#web-application-flow
We don't need to make this property public after all
Do not lower case user supplied value

That value might legit need upper and lower case such as the state in an
oauth call.
Implement observable oauth client

Rx will never die
Fix up the mono projects
Return Uri to appease convention tests
Add DebuggerDisplay attributes
Add comments and debugger display
Merge pull request #462 from octokit/haacked/oauth

OAuth Web Flow methods