Added multi-name-imports to the lark.lark grammar.

Include relative imports and rule rewrite

Without BIN_NUMBER, the following error occurs:
lark.exceptions.UnexpectedCharacters: No terminal defined for 'b' at line 1 col 2

0b10
 ^

* All exceptions are now under exceptions.py
* UnexpectedInput is now superclass of UnexpectedToken and UnexpectedCharacters,
  all of which support the get_context() and match_examples() methods.

Nearley tool still needs fixing

* 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.

(inspired by issue #116)

- Reasoning addressed in Issue #116
- Renamed example and grammars file to use new extension.
- Changed `.g` to new extension of `.lrk` in places where it is
  referenced.