Add kew to benchmarks

Results on my machine:

```
Promise overhead (concurrent calls) x10000:

1:   278ms  Base (plain Node.js lstat call)
2:   283ms  kew: Dedicated wrapper
3:   410ms  Deferred: Dedicated wrapper
4:   496ms  When: Dedicated wrapper
5:   499ms  Deferred: Promisify (generic wrapper)
6:   542ms  Deferred: Map + Promisify
7:   969ms  Q: Dedicated wrapper
8:  1265ms  jQuery.Deferred: Dedicated wrapper
9:  1761ms  Q: nbind (generic wrapper)
```
and

```
Promise overhead (calling one after another) x10000:

1:   338ms  kew: Dedicated wrapper
2:   371ms  Base (plain Node.js lstat call)
3:   523ms  When: Dedicated wrapper
4:   537ms  Deferred: Dedicated wrapper
5:   598ms  Deferred: Promisify (generic wrapper)
6:   612ms  jQuery.Deferred: Dedicated wrapper
7:   867ms  Q: Dedicated wrapper
8:  1240ms  Q: nbind (generic wrapper)
```