HHVM and PHPUnit Compatability

This update fixes a fatal error that occurred when the test suite was run on HHVM. The bug was caused due to the way Mock objects get generated and an incompatibility with HHVM's handling of static objects, so this release simply works around the need for using the mock driver in the first place. Some additional compatibility changes were also made. 

This pull requests results in a test suite that runs all the way through, although there are still some failures (all of which are related to the session handling portion of Stash). This is an extremely important step for HHVM compatibility.