Improve warning messages coming from the CHARMM parsers.

If a dihedral type was 'replaced', this warning used to be issued:

```
/usr/local/openmm/dev/openmm/python2.7/simtk/openmm/app/charmmparameterset.py:347: UserWarning: Replacing dihedral <simtk.openmm.app.internal.charmm.topologyobjects.DihedralType object at 0x7f7249df2150> with <simtk.openmm.app.internal.charmm.topologyobjects.DihedralType object at 0x7f7249d2d7d0>
```

This is a pretty useless message.  With this change, it now reads:

```
/usr/local/openmm/dev/openmm/python2.7/simtk/openmm/app/charmmparameterset.py:347: UserWarning: Replacing dihedral <DihedralType: k=0.14; phase=0.0; per=3> with <DihedralType: k=0.0; phase=0.0; per=3>
```