Left recursion + general cleanup

• New support for left recursion with correct associativity. All
  test cases pass.
• Left recursion is enabled by default. Use the
  @@left_recursion :: False directive to diasable it.
• Renamed the decorator for generated rule methods to @tatsumasu.
• Refactored the tatsu.contexts.ParseContext for clarity.
• The @@ignorecase directive and the ignorecase= parameter no
  longer appy to regular expressions (patterns) in grammars. Use
  (?i) in the pattern to ignore the case in a particular pattern.
• Now tatsu.g2e is a library and executable module for translating
  ANTLR grammars to TatSu.
• Modernized the calc example and made it part of the documentation
  as Mini Tutorial.
• Simplified the generated object models using the semantics of class
  attributes in Python.