ES6 style generator

I just modified generator syntax of JSX to give some compatibility with ES6 (draft).

# syntax
```
// internally compiled to a function of type `() -> Generator.<type>`
function * foo () : type {
  ...
}
```

# compiler option

A new compiler option `--(enable|disable)-generator-emulation` is introduced, which flag switches on and off the generator transformation to legacy ES5 compatible code. The options is turned off by default.