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
I suggest that we keep `ScalarSymbol` in `interface.py` and out of `parser.py` and then move `exprify` into `parser.py` to remove the dependency between `interface` and `parser`