Add git branch option and polling interval for Wordpress charm

The current option only downloads from the git once. Every time the git repo is updated we have to manually remove the git repo address save it as blank and then readd it to be able to pull the new git revision. It should have either a button to do a pull now or have a cron for automatic polling of the repo.

It would be nice to have a branch option so the same repo can be pushed to two different wordpress services. For example a live site and a development sandbox site.