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
Wrap Milestone in an object to display extended information

Use Milestone.__str__ in scc milestone list
Add milestone delete command and use command logger
Move get_milestones() and get_milestone() to GitHubRepository class
Fix description setting in `scc milestone create`
Add close subcommand to scc milestone
Add `scc milestone update` subcommand

- Add MilestoneCommand.update() method
- Refactor permissions check into MilestoneCommand.check_write_permissions()
- Define methods for grouping arguments (title, description/date)
Add implementation for setting the due date at milestone creation/edition
Add date formatting to the help in the milestone subcommands
Merge pull request #157 from sbesson/milestone

Milestone command