Add probabilistic matrix factorization

[Danny]

I implemented probabilistic matrix factorization last night and pushed
the changes to github. This looks to be the easiest model to get
working at the moment, and it's now set as the default.

My immediate next TODOs are as follows (unless somebody else wants to
jump in and do them -- let me know):
- add functionality to Bracket, so that it can take as input a trained
model, then greedily predict the outcome of each game to arrive at a
full bracket.
- add scoring function to Bracket, so the result from above can be
scored against the true bracket outcome (note: the true bracket
outcome is already constructed while loading data)
- add a proper cross validation framework