Fleshing out visitor code.

Adds the following:

- Improved context handling and parsing with completed tests to match 1.0.0 of the JavaScript version
- Matching block unit testing to match 1.0.0 of the JavaScript version
- Added Helpers, which differs from the 0.0.x implementation. It's documented in detail in NOTSUPPORTED.md
- Merged in cross build sbt changes from master / Gregor