parse scene numbers at the end of scene headings

This, I hope, is the first PR of several that covers one specific element of the Fountain spec.

Changes unrelated to scene numbers:

* organize specs more
* commit to using the `should` syntax in rspec (close #16)
* include status report checklist in README
* switch back to rubygems version of parslet, as it's been updated
* remove ruby 2.1.0 from .travis.yml -- I think that's fine?