Add support for 'phpunit_binary' config setting

Summary:
If present, this will override the default phpunit path. Allows easy
integration of Composer-provided installs of phpunit (ex. set phpunit_binary to
vendor/bin/phpunit). If the path provided doesn't resolve to an executable it
will assume the path is relative to the project root.

fix line length issue

Test Plan:
Added phpunit_binary to .arcconfig in a simple project using Composer with
phpunit/phpunit package as part of require-dev (installed to
$ROOT/vendor/bin/phpunit). Phpunit not otherwise installed on the system. Set
unit.engine to PhpunitTestEngine. Confirmed that 'arc unit' used the specified
binary, both at project root and from subdirectories.

Reviewers: epriestley

CC: aurelijus, Korvin, aran

Differential Revision: https://secure.phabricator.com/D6791