Reimplement Zones to improve performace and API

* Remove Gate API in favor of Zone.bindCallback
* Convert all library overrides to use new bindCallback APIs
* Reduce Zone library overhead
    * Basic HTTP overhead reduced from 60% to 18%
    * Basic Express app overhead from 50% to ~28%
* Update test cases to use nodeunit
* Add jsDoc for public facing APIs
* Add promise like zone.resolve and zone.reject APIs
* Add examples doc
* Add basic http server benchmarks
* Bump version to 0.2.1-pre

Fixes #12
Fixes #10