Converted the array containing atom block indices for the neighbor list from ushort2 to int.  This removes the hard limit of 2 million atoms.
Merge branch 'master' of https://github.com/peastman/openmm
Merge branch 'master' of https://github.com/SimTk/openmm
Python wrapper for getVirtualSite() returns the correct subclass
Fixed bugs in addExtraParticles()
Merge pull request #24 from peastman/master

Various bug fixes and code cleanup