Add support for harmony class static member functions

Support for ES6 class static methods was added to the spec draft on March 8:
http://wiki.ecmascript.org/lib/exe/fetch.php?id=harmony%3Aspecification_drafts&cache=cache&media=harmony:working_draft_ecma-262_edition_6_03-08-13-rev14markup.pdf

This adds support for a static token before class methods by adding a new
`kind` value of "static" in the Syntax.MethodDefiniton node interface.

I've added a couple of new tests and verified that all tests still pass.
I've also confirmed that running the benchmark suite results in no significant
perf regressions.

https://code.google.com/p/esprima/issues/detail?id=406