Added git mode support for merging, rebasing, and bisecting

<B> Designates Bisecting
>M< Designates Merging
>R> Designates Rebasing
Remove mailcatcher

The mailcatcher gem does not work correctly when executed through Bundler.
Merge pull request #2685 from delynn/patch-2

Remove mailcatcher from bundler plugin
Merge remote-tracking branch 'origin/master' into Darwin
Update git prompt info

Use ``status'' to parse git dirty instead of ``diff-index''
Merge pull request #2633 from redsunsoft/agnoster-theme-git-mode-addition

Added git support for merging, rebasing, and bisecting to agnoster theme
Fix for Python3

In Python3 without universal_newlines set to True
output from Popen was byte-encoded.
Fix `docker rmi` tab completion

This is exactly what happens on tab completion for docker rmi. This commit
fixes that.

```sh
$ docker rmi <hit TAB>
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
```
Merge pull request #2693 from javiertejero/master

Fix `docker rmi` tab completion
Merge pull request #2729 from grodzik/master

Fix for Python3
Merge remote-tracking branch 'origin/master' into Darwin