Rename Rugged::Repository#file_at -> blob_at
Rename Rugged::Repository#file_at -> blob_at

This method now returns a Rugged::Blob object instead of just the
content.

libgit2/rugged,libgit2/rugged,tpickett66/rugged,tpickett66/rugged,Acidburn0zzz/rugged,libgit2/rugged,spraints/rugged,tpickett66/rugged,Acidburn0zzz/rugged,saraid/rugged,saraid/rugged,spraints/rugged,Acidburn0zzz/rugged,saraid/rugged