This drops the branch option.  The version option is overloaded
to mean either a sha1, branch, or tag.  This also adds the option
'remote' which defaults to 'origin'.
clone() was simplified by removing the checkout operation.  That
happens later when switch_version() is called.

Added the methods get_branches(), is_remote_branch(), and
is_local_branch().  get_branches() returns an array listing all
of the branches for the git repository.  is_remote_branch() checks
whether the arguments supplied correspond to a remote branch.
Similarly, is_local_branch() checks for a local branch.

The pull() method now checks to see if it is on the desired branch.
If not, it checks out the requested branch and then does a pull.
This should keep issue #604 still fixed.

switch_version(), formerly switchver(), looks to see if it is
checking out a branch.  If a branch, it checks it out with the --track
option.  This type of checkout was in pull() before.

Updated pull, clone, and switch_version to return (rc, out, err).