Add AlreadyMerged command

`scc already-merged` now provides a way to check which branches both local and
remote can be removed. Anything not listed should be inspected for removal.

Example:

```
@already-merged ~/code/scc.git $ ./scc.py already-merged origin/master refs/heads
...
2012-12-17 00:57:39 -0800 master   --- Merge pull request #23 from joshmoore/fix_logging
2012-12-17 10:01:54 +0100 check_pygithub   --- Remove static FACTORY in favor of create_instance
2012-12-17 10:08:07 +0100 logging_fix   --- Add logger to PullRequest class
2012-12-17 10:54:48 +0100 fix_logging_2   --- Fix logging by setting right global variable
2012-12-17 12:40:18 -0800 already-merged   --- Merge pull request #28 from joshmoore/fix_logging_2
```

Usage questions:
 * default to the current branch rather than specifying the target (here `origin/master`)
 * allow specifying the format and the ordering?
 * other "better" defaults?
 * encourage users to fetch first?
 * allow printing out courses of action? e.g. `git branch -d foo`