Include ffxml scripts in OpenMM repository

Previously, scripts to convert other forcefields into the OpenMM `app` layer `ffxml` format lived in a separate repository:
https://simtk.org/websvn/wsvn/pyopenmm/trunk/scripts/appscripts/#_trunk_scripts_appscripts_

This has proven confusing and unnecessary.  The total space required is only 3.9M, which is a negligible change to the repository size (already ~160M).  

This PR imports these scripts into the main OpenMM repository, where they can be more easily found, maintained, and extended.

This PR addresses issue #490.