end-to-end release script

It would be very useful to have a script which uses the distro file on github to lookup, clone, run bloom, push, and open a pull request for a given package all in one sweep.  Something like this:

```
$ bloom-release --ros-distro hydro foo [<trackname>]
Finds foo-release in ros/rosdistro:releases/hydro.yaml...
Clone foo-release to /tmp/...
Run git-bloom-release <track name> in foo-release
Push foo-release
Author pull request to ros/rosdistro with new version info
```

This would be particularly useful when releasing lots of packages at once (like for hydro).