Remove Git Plus: prefix

When I open `git-plus:menu` to call `git-plus:push` in current master branch

![2014-06-15 16 36 41](https://cloud.githubusercontent.com/assets/557961/3280402/03471c82-f460-11e3-88c8-29c10c77bbfe.png)

It is matches `Git Plus` words. This is very inconvenient. So I removed `Git Plus:` prefix.

![2014-06-15 16 41 23](https://cloud.githubusercontent.com/assets/557961/3280412/96b62fd0-f460-11e3-835f-f209af7715d3.png)

Now I can match `git-plus:push` by one word `p`.

thanks.