Add comments to much of Brushfire core

I'm basically a complete Scala newb, so I decided to run through the repot and see if I could figure out what everything was doing.

I ended up making comments for everything. If something's wrong let me know and I'll fix it.

This is probably a good smoke test to see how readable the code is to complete outsider, with no experience in Scala, Scalding, or Algebird.