Add __repr__ to app's enums, so that we can print them easily

Right now, you don't get very useful information if you print out enumerated constants in the python app. The use case for this is in a python script where you'd like to report to the user what openmm api calls are being executed, or what options they've selected.
 
```
In [1]: print app.PME
<object object at 0x7f7592100770>
```