Set commit time from command line (using -t)

Uses existing CommitOp.setCommitterTimestamp method to set commit time to a Unix timestamp from the command line, adding a -t option.

For example, creating an historic commit on October 26, 1985.

geogit commit -m "first time-traveling commit" -t 499152060000

The web API returned this response in commit history:

![screen shot 2013-07-16 at 2 22 34 pm](https://f.cloud.github.com/assets/643918/806959/5f7c44a4-ee45-11e2-8dcb-594008c20163.png)