Create a git module to interface with Git, and update the readme

The Git module works on the current folder, and can perform the following
functionalities:

- `clone(url)` - clone a repo, removes previous repos in this folder
- `checkout(commit_id)`- checks out a certain commit
- `current_commit()`- gets the current commit_id
- `commits()` - get a list of all the commits {commit_sha,commit_title}
  in chronological order
- `commit_message(commit_id)` - gets the full commit message from a certain
  commit

All `commit_id` instances are shortened to 7 characters.