Adding ApiGen support for Robo, with tests

Recently starting using Robo but couldn't help but notice a lack of a documentation generator task built-in. Added a task for using ApiGen (http://apigen.org/) with all supported CLI flags, and associated unit test.
Dawning on me this isn't really required; it's just that I modeled the ApiGenTask after the PHPUnit task which ensures it can locate a phpunit executable. For purposes of a unit test, I should probably find another means of specifying (but not verifying) the apigen path.