type stubs: A consistent type for `Token.value`

The Token's value is described as being a in the attribute list: 460a221923/lark-stubs/lexer.pyi (L84) and as `Any` in `__init__` parameters:  460a221923/lark-stubs/lexer.pyi (L91)

Yet one of the recipes, [Use a transformer to parse integer tokens](https://github.com/lark-parser/lark/blob/master/docs/recipes.md#use-a-transformer-to-parse-integer-tokens), recommends to set it to other types. So IIUC, `Any` is the correct one.

v_args is described as taking a callbable as argument: 36a7b050c1/lark-stubs/visitors.pyi (L75-L79)
Yet the documentation states it can decorate a class: 5b30ba4841/lark/visitors.py (L417-L418)

- `user_repr` is now a method
- Fix for python 2.7
- excepts -> expected

 Conflicts:

	lark/common.py

	lark/exceptions.py

	lark/lexer.py

	lark/load_grammar.py

	lark/parser_frontends.py

Refactored parser_frontends. Now significantly simpler

Fix tests failing when used with `python -m unittest discover`

 Conflicts:

	tests/test_parser.py

Note: This refactor opens the door for implementing a ContextualLexer for Earley.
But unlike the existing one for LALR, it will have to be computed at runtime,
rather than ahead of time.

regression-fix for #760

Fixed pyparsing link to point to 

https://github.com/pyparsing/pyparsing

See https://gitter.im/lark-parser/Lobby?at=5fb83369771c185e0eb8c0e2

mypy thinks that Token's __init__ is inherited from __str__(). That's
not wrong -- it's just irrelevant, because Token also implements
__new__(). Token's _effective_ constructor signature is determined by
its __new__() method, so that's what I have used in the type hint.

Not clear if 'value' is supposed to be Any, but that's what I need in
my application. And it works just fine!

The code is crystal clear:

  assert lexer in ('standard', ...) or issubclass(lexer, Lexer)

But the type hint said that lexer must be an _instance_ of Lexer, not
a subclass. This change fixes it to require a subclass of Lexer.

Adding missing Testcase to `__main__`

Fix IndexError