Moved charges.py to openeye.py

1.  Renamed charges.py as openeye.py
2.  Moved `molecule_to_mol2()` from `utils.py` into `openeye.py`.  
3.  This means all gaff2xml functions using OpenEye tools are in `openeye.py`.  I think this will help keep things organized.  

Regarding (2), I left a symlink to the old location for backwards compatibility.