Add `--watch` and `--restart` support

* `--watch` causes the process to wait for the path given as its argument
  to exist before starting up the session acquisition.
* `--restart`, which only works with `--watch`, causes any change to the
  watched path to trigger a restart of our command, if it is running.
Add support for `--verbose`

* Turns on debugging, the same as setting `DEBUG=consul-elected`
0.0.3: Add grunt build; move coffee-script to a dev dependency