Implement a scalar expression parser

closes #395

This will fail until the `py.test` PR (#384) goes in

- [ ] string tests
- [x] failing (they should) chained comparison tests
hm chained comps in blaze expressions are not behaving how i might expect them to:

```
ipdb> p x, y, z, w
(x, y, z, w)
ipdb> x < y < z
y < z
```