Refactor function parameter handling.

This is the preparation to support default value for function parameter.

https://code.google.com/p/esprima/issues/detail?id=519
ES6: Default Parameter Value (for functions).

https://code.google.com/p/esprima/issues/detail?id=519
Document the support for ES6 Default Parameter Value.

https://code.google.com/p/esprima/issues/detail?id=519
ES6: Default parameter for arrow function expression.

https://code.google.com/p/esprima/issues/detail?id=517
https://code.google.com/p/esprima/issues/detail?id=519