Implement a scalar expression parser

closes #395

~~This will fail until the `py.test` PR (#384) goes in~~

- [x] simple number tests
- [x] python3 parses expressions like `-1` different than python2 need to investigate
- [x] string tests
- [x] failing (they should) chained comparison tests
Works for me.  Mostly I want to convey that a lot of this was thrown together and so bigger changes are welcome if you feel the need.