Milestone command

This PR should allow to manipulate GH milestones via `scc`. At the moment, two commands are implemented:

- `scc milestone list`: List all open milesontes in the repository and its submodules
- `scc milestone create`: Create a new milestone in the repository and its submodules, e.g.

  ```
  scc milestone create 5.0.2 --description 'See http://trac.openmicroscopy.org/ome/roadmap/%s' 
  ```

Other points to consider:
- [ ] add more information in `scc milestone list` (date, description)
- [ ] allow additional parameters to be passed in `scc milestone create`
- [ ] add a `scc milestone update` function to update existing milestones?

/cc @joshmoore