Added support for git blobs

Added API support for http://developer.github.com/v3/git/blobs/

I'm eager to contribute to this project, but I'm still getting acquainted with the code (as well as Go).  I've tried to mimic the style of the code as best I could.  If there is anything I can change, please let me know.

I'm looking forward to contributing more to this project in the future.

Thanks,
-Matt