Version 2.0 beta

* Refactored PhantomJS support to utilize GhostDriver, instead of
  PhantomJSPlease
* Refactored UAC design to be more stable and avoid wasting resources
* By default, Venus will now ignore all test files that do not contain
  at least one @venus-* annotation
* Switched from webdriverjs to selenium-webdriver NPM module for
  interfacing with webdriver API