Add magit-gerrit-download-review

This will checkout the review into a branch called :

review/%(username)/%(topic)

Add a function magit-gerrit-create-branch-force to be able to checkout
the most recent review to an existing branch.

This mimic the behavior of gerri-review CLI tool.

Signed-off-by: Chmouel Boudjnah <chmouel@chmouel.com>
Merge pull request #11 from chmouel/add-download-review

Add magit-gerrit-download-review