Merge pull request #5 from SimTk/master

updates
added remove_solvent
Merge branch 'master' of github.com:cxhernandez/mdtraj into rm-solvent
added copy of solvent to avoid issues
added checks
changed default to None
created residue_names.py
license stuffz
Merge pull request #589 from cxhernandez/rm-solvent

Remove solvent ions