1) Failed to detect newlines in regexps of the form [^...]

2) Last token didn't get end_line & end_column

* 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.

I found this while porting Token to C, essentially the value and pos_in_stream members of Token were swapped in ``__reduce__``, which means running ``pickle.loads`` and ``pickle.dumps`` would result in unpickled tokens whose value was the original's position in stream, and vice versa. In my C extension this caused a TypeError exception, but the behavior will have to be corrected in both.

Added __slots__ to RulePtr and Token, resulting in significantly lower memory consumption.

As suggested by @drslump.