Nearley tool still needs fixing

Added Tox support (for travis testing)

Small collisions

Anonymous tokens would become visible if they had the same value as named tokens.
That's because they are merged for the lexer. But after this change, the rules for
visibility are based on their use in the rule, and not their name or identity.

* Removed the scanless parsing feature (dynamic lexing is king)
* Default LALR lexer is now contextual

- @erezsh added the new Lark reference and some other things using the
  current .g extension, so I wanted to resolve them for PR #117
- Renamed lark.g to lark.lark.
- Changed lark_grammar.py to use .lark file extensions.
- Changed urls that used .g to use .lark.

- Merging updated upstream into branch for file extension changes.
- Will push so Pull Request has no remaining conflicts.
- Also will change the file type of lark example grammar.

(inspired by issue #116)

Interpreter class: fixed default behavior to return results