Add git versioning to Arethusa

Every build now knows about the commit it's build of, as well as about its build time.

This is done through a Constant which is written by the `grunt version`. This constant is taken and exposed by the `versioner` service. The `helpPanel` contains an `About` section which displays this information. During the process of doing this, the `helpPanel` was refactored through extraction of a couple of directives.