Support floats in CQL

Filtering & sorting by a float value:
![screen shot 2014-12-05 at 5 19 08 pm](https://cloud.githubusercontent.com/assets/98301/5323926/f16978cc-7ca2-11e4-9083-cee01b9138bb.png)

I implemented this by grabbing column types from the VCF spec on each `/genotypes` call. To avoid redundant database fetches, I memoized the `spec()` function.

Fixes #344