Initial add of the generic optimizer code

Before digging into this code, look at the Rules implemented at the bottom of DagOptimizer.

There are several TODOS here:

- [x] After I split the code into separate files, I started hitting a scala 2.9 compiler bug, that I don't hit in 2.10. I'll try to work around, or possibly hope that Twitter finally gets on 2.9 before we need this.

- [x] Tests tests tests. I played around at the repl, but clearly we need super hardcore tests for any optimization framework.

- [ ] Actually call this code from an existing platform.
I am a bit wary of covariance in frameworks with deal heavily in serialization. I know this is common in Scalding etc, but I was just curious your thoughts on it. Giong to keep an eye out later in the review, I'm just worried that especially when paired with implicits, it's possible that people could pick up a Semigroup, for example, of an unintended type.