Added initial Tests 

I also added a function in Ontology class that gets an entity by URI (String) to help with testing.
There is a either a problem with my function or how URIs are handled for individuals (see commented out code + comment). 

I am happy to fix that before you accept the PR. I just wanted to let you take a look at it so far to see if you saw something obviously wrong.

Also, I realize that testing the entities.py functions is probably overkill since it would most likely all be tested by testing ontology.py functions. But I just wanted to standardize how we are going to test things by testing the smallest file first.