Fix: rewrite no-unused-vars rule (refs #1212)

Before:

```
$ npm run perf

> eslint@0.7.4 perf /Users/michael/projects/eslint
> node Makefile.js perf

CPU Speed is 2600 with multiplier 7500000
Performance Run #1:  1299.1717899999999ms
Performance Run #2:  1338.114744ms
Performance Run #3:  1288.453107ms
Performance Run #4:  1292.191793ms
Performance Run #5:  1307.2589520000001ms
Performance budget ok:  1299.1717899999999ms (limit: 2884.6153846153848ms)
```

After:

```
 npm run perf

> eslint@0.7.4 perf /Users/michael/projects/eslint
> node Makefile.js perf

CPU Speed is 2600 with multiplier 7500000
Performance Run #1:  1165.686332ms
Performance Run #2:  1185.946108ms
Performance Run #3:  1170.5925320000001ms
Performance Run #4:  1173.558406ms
Performance Run #5:  1189.729579ms
Performance budget ok:  1173.558406ms (limit: 2884.6153846153848ms)
```

Ref. #1212.