Return the VersionControl superclass from a string, for example
        VersionControl.from_string('git') will return Git.

def from_string(vc):
        """Return the VersionControl superclass from a string, for example
        VersionControl.from_string('git') will return Git."""
        try:
            # Note: this means all version controls must have
            # a title naming convention (!)
            vc = globals()[vc.title()]
            assert(issubclass(vc, VersionControl))
            return vc
        except (KeyError, AssertionError):
            raise NotImplementedError("Unknown version control system.")