Add note about browser compatibility to README

Closes #105
ecma 6 partial support: let and const
Note partial es6 support in docs
Split up `parseStatement()`

It was purely interpreted by the V8 engine. Splitting it up allows the
individual pieces to be optimized, so less is interpreted.
Mark version 0.6.0
Bump version number post 0.6
Fix another bogus piece of code from 31e77b975ed0bf14f68f9b0f6608bbe78d694bf0
Started porting JSX support from esprima-fb.