Implemented Save and Unsave from Submission

Hey all,

I'm trying to get a feel for the codebase/git (this is my first github contribution!), so I've implemented these two methods to start.

The only thing I'm confused on is how to handle the 'category'.  I think it's correct, but I'm unsure how to test.  I don't have Reddit gold, so I didn't even know this was a feature.  I left the test blank for now, but I will go back and change once I figure it out.   Any ideas?

I based my unit tests on previous ones.

Thanks!