Added `[merge]` keyword in prompt when a merge is in progress

As brought up in #25, a merge can sometimes be clean which makes it seem like the prompt is `git status` is normal. To remedy this behavior, various avenues were explored (e.g. new symbols, alternate colors). However, one seemed to fit in well with the current prompt:

![screenshot from 2014-02-28 02 25 00](https://f.cloud.github.com/assets/902488/2292898/99d26dca-a062-11e3-97f9-9fc9be7ebe57.png)

In this PR:

- Added `get_git_progress` to prompt which outputs ` [merge]` if a merge is occurring
- Added `get_git_progress` to prompt, defaulting to red
- Added `PROMPT_GIT_PROGRESS_COLOR` as an available color override
- Added tests for `get_git_progress`
- Updated prompt tests to reflect `get_git_progress's` addition

@rpdelaney Can you please look over the changes and approve/disapprove? Following this PR's merge, we can follow up with another one that adds all missing states (e.g. `rebase`, `cherry-pick`) from the `git` code referenced in #25.