Closes #305.

- Makes rule ordering the default ambiguity tie breaker.
    E.g.
    start: a | b
    a: "A"
    b: "A"

    will return:
    start
        a

    start: b | a
    a: "A"
    b: "A"

    will return
    start
        b

- Replaces the ambiguity='resolve__antiscore_sum' with a separate option: 'priority'.
    The priority option has 4 values: 'auto', 'none', 'normal', 'invert'.
    'Auto' maps to 'Normal' for CYK and Earley and 'None' for LALR.
    'None' filters your priorities and ignores them. This saves some extra tree walking on Earley.
    'Normal' uses your priorities untouched, mimicing the old behaviour.
    'Invert' negates your priorities, emulating the old 'resolve__antiscore_sum' behaviour.

    This allows you to use priority logic even when ambiguity=='explicit', to get a better idea
    of the shape of your tree; and to easily disable priorities without removing them from the
    grammar for testing (or performance).

- ambiguity='explicit' now correctly returns an ambiguous tree again, as 0.6 did.

Added tox.ini posargs option, you can pass arguments to unit tests

https://stackoverflow.com/questions/35697353/how-to-run-only-one-test-in-tox

Added .tox to .gitignore (to use tox locally)

Fix misspelling on lark/lark.py documentation

Avoids recursion and improves performance slightly

Update from latest 0.6