PHPUnit test engine

Summary:
PHPUnit wrapper for arc.
The idea here is simple - find the test case which is related to the updated
class file. Generate tmp files for json & clover reports, run phpunit
with provided arguments.
It supports phpunit configuration file setting in .arcconfig: phpunit_config.
Path should be relative to project root.

Test Plan:

• Set unit_engine to PhpunitTestEngine
• Try running tests with & without phpunit_config option.

Reviewers: epriestley, davidreuss

Reviewed By: epriestley

CC: aran, Koolvin, jungejason

Differential Revision: https://secure.phabricator.com/D2472