Show message when local branch doesn't match remote

I saw this issue (#9) last night and thought this would be a fun challenge. I added a message message when the local copy is either behind or ahead of the remote branch. When behind it links to github with a commit comparison to show the changes.

Here's an example of each message.

![behind](https://cloud.githubusercontent.com/assets/241816/4872544/98342050-61e9-11e4-9789-51c33cb4aee3.png)

![ahead](https://cloud.githubusercontent.com/assets/241816/4872528/3d49f016-61e9-11e4-87ee-72a3da939c26.png)

This was my take on adding this feature and I know there are multiple ways to achieve this. If you like it great, if not, that's great too. Just thought I'd share this fork.