Initial cut of the ES2015 parser

- This is basically identical to the ES5 parser at this point, with the
  references modify to reflect the ES2015 intent that this parser will
  handle.
- Additional changes since the initial cut have been squashed into this
  commit to match as close to the current implementation and tests,
  including:

  - Fix to disambiguation of DIV vs. REGEX lexer tokens.

src/calmjs/parse/asttypes.py

@@ -157,6 +157,10 @@ class ES5Program(Program):
     pass
 
 
+class ES2015Program(Program):
+    pass
+
+
 class Block(Node):
     pass