Add `estimate` command, and fix bugs

1.8.0
- Add the `estimate` command. Example: `pivotal-slacker estimate 123 --points 1`
- Shell mode no longer bails out when you run a command incorrectly (e.g. with missing required options).
- Reports any errors encountered when running the `start` command. Now when you try to start an unestimated feature, you'll get an error message about it.