Implemented expand_by_class. Closes #362.

 Optional argument separators requires changes Classify and FindByClass (not currently implemented for find_by_class either), will be implemented later. Will require implementing Py_find_by_class manually.
This is good, we need to do more python side testing. I'd like to do testing of the code via go test also though as the backend package doesn't depend on backend/sublime and `backend/sublime` could in theory be compiled out. I think that the way the coverage tool works, it treats packages as separate entities. In other words, even though this test does touch upon the new `backend.View.ExpandByClass` function, it only reports coverage for the `backend/sublime` package and not the new function itself. 

Do you have time to look into writing the go side test also? If not, lets merge and open up a new issue number for someone to take a look at it.