Add new Milestone command to scc

- use argparse sub_parsers to define `scc milestone COMMAND`
- add a flag to Command.__init__ to prevent setting the default command
- add first implementation of `scc milestone list`
Add milestone creation implementation
Add permissions check for scc milestone create
Add missing blank line
Merge pull request #156 from sbesson/description_comment

Add comment for PullRequests without a description
merge master -D org: PR 157 (Milestone command)
merge master -D org

Repository: openmicroscopy/snoopycrimecop
Already up-to-date.

Merged PRs:
  # PR 157 sbesson 'Milestone command'

Generated by SCC-merge#288 (http://ci.openmicroscopy.org/job/SCC-merge/288/)