Simplifying keywords code and improving a bit its speed.

Running the benchmark script I could get around 3% improvement. Not big, but at least
the code was simplified and the speed didn't decrease.

Previous code:
```
$ npm run-script benchmark

> esprima@1.1.0-dev benchmark /Users/fabs/Dev/other/esprima
> node test/benchmarks.js

    Underscore 1.5.2    42.5 KiB      8.93 ms ± 2.85%
      Backbone 1.1.0    58.7 KiB      9.74 ms ± 2.02%
      MooTools 1.4.5   156.7 KiB     71.05 ms ± 6.14%
        jQuery 1.9.1   262.1 KiB    101.38 ms ± 5.30%
          YUI 3.12.0   330.4 KiB     89.51 ms ± 6.15%
 jQuery.Mobile 1.3.2   350.7 KiB    159.50 ms ± 9.59%
       Angular 1.2.5   701.7 KiB    146.68 ms ± 5.62%
                     ------------------------
                      1902.8 KiB    586.79 ms
```

With the change:
```
$ npm run-script benchmark

> esprima@1.1.0-dev benchmark /Users/fabs/Dev/other/esprima
> node test/benchmarks.js

    Underscore 1.5.2    42.5 KiB      9.87 ms ± 1.97%
      Backbone 1.1.0    58.7 KiB     11.92 ms ± 3.53%
      MooTools 1.4.5   156.7 KiB     80.74 ms ± 7.43%
        jQuery 1.9.1   262.1 KiB    102.01 ms ± 4.99%
          YUI 3.12.0   330.4 KiB     80.69 ms ± 2.32%
 jQuery.Mobile 1.3.2   350.7 KiB    135.08 ms ± 3.14%
       Angular 1.2.5   701.7 KiB    139.18 ms ± 3.37%
                     ------------------------
                      1902.8 KiB    559.50 ms
```

Ran the tests and everything is fine.
No coverage regression.

https://code.google.com/p/esprima/issues/detail?id=498