Added packaging functionality to install script.

These changes fix the addin XML file to support packaging using ``mdtool`` (because apparently the tags in the XML are case sensitive).  It also patches the install script to allow automatic packaging using ``cobra install.cobra -- package``, and adds a usage message when one of ``/?``, ``/h``, ``-h``, or ``--help`` is supplied.

As far as testing, I've done basic testing on Mac/Windows/Linux for both building and installing the addin from the created package.  It appears to work cross-platform correctly, but particularly the Windows case could be better-tested (my development environment under Windows is not as stable).