Standalone CSS for embedding the MyUSA button

Updated the styles in the partial to reuse as much as
possible.  Changed the classes to have a .myusa-button.white
combination of classes instead of .myusa-button-white as
its own class.

Added the raw button.css and minified button.min.css
(using cleancss from nodejs), which is also mirrored
at https://s3.amazonaws.com/myusa-static/

You can now use in your HTML

<a href="" class="btn btn-social btn-myusa">Connect with MyUSA</a>
or
<a href="" class="btn btn-social btn-myusa white">Connect with MyUSA</a>
Add API documentation for using the MyUSA button

Move the OAuth stuff under authentication too.
Merge pull request #397 from 18F/feature/developer-button

Implement and provide docs for the MyUSA login button