Extend model family to random forests.

[dcrankshaw]

To evaluate the implications of more complex feature functions we should add random forests to the model family. More specifically we need to:

• specify how random forests fit in the model family (this should be trivial)
• define how to update the random forest models offline
• implement random forests for some more traditional prediction task

[jegonzal]

@henryem let's discuss this issue more when you get a chance.