Performance improvement

Hi,

In js it is (much) faster to access object properties statically than dynamically.

So, coding:
```javacript
var then = 'then';
var a = this[then];
```
Will give the same result as
```javacript
var a = this.then;
```
Only the second style will be aprox. 3 time faster (when running with V8)

See this: http://jsperf.com/lookup-defined-vs-undefined-properties/5

Yoni.