Mark |parse| and |SyntaxError| as internal identifiers

The |parse| function and the |SyntaxError| exception were meant as
internal, so let's mark them as such.
Expose the parser object in action/predicate code

The action/predicate code didn't have access to the parser object. This
was mostly a side effect actions/predicates being implemented as nested
functions, in which |this| is a reference to the global object (an ugly
JavaScript quirk). The initializer, being implemented differently, had
access to the parser object via |this|, but this was not documented.

Because having access to the parser object can be useful, this commits
introduces a new |parser| variable which holds a reference to it, is
visible in action/predicate/initializer code, and is properly
documented.

See also:

  https://groups.google.com/forum/#!topic/pegjs/Na7YWnz6Bmg