Add more relationships to the activities endpoint.

@jbirdjavi 
I'm not totally sure how you wrote this without your head exploding, but let me make sure I know what you're going for here...
I take it you're adding complexity on the backend in order to reduce complexity with the API...  e.g. If someone adds an include parameter in a request, it automatically includes some relationships underneath it as well.

Given the complexity of this method, I'd like to see specs for all the various possibilities that could be passed into this method.

Or if it's possible to do without some of the recursion, I think that would make this easier to read.  Ideally, for readability sake, it'd be nice if only one possible parameter combination could be passed into this method.  Recursion + multiple parameter type possibilities makes this method an absolute disaster to read.  :)