Contextual Standalone, import rules, and many bugfixes

New features:

• Standalone parser now uses the contextual lexer

• Experimental support for importing rules in the grammar

Bugfixes:

• Fixed a hidden bug in the LALR algorithm (no, it probably doesn't affect you, see issue #250)
• Fixed v_args handling of staticmethod and classmethod (Issue #246, #249)
• Many fixes to the standalone parser
• Fixed propagate_positions (and added start_pos/end_pos attributes to Tree.Meta)
• Fixed automatic terminal naming
• EOF token now gets proper line/column number

Also improved error messages and documentation