Resolve Memory Leaks #17

This series of commits fixes the memory leaks described in issue #17.

* ckennelly/benchmark@fa908926c7ab9270037e2ab71036f8f6f87e9850 addresses the memory leak that occurs when we fail to free compiled regular expressions.  It additionally adds a unit test to exercise the new wrapper.
* ckennelly/benchmark@9f27edbb1650c297f511c4b52e563ee3f0619348 addresses the indirect memory leak that occurs when we fail to destroy benchmark instances.

I've proposed adding an `ExternalProject` reference in CMake to Google Test.  I have an alternate series of commits (ckennelly/benchmark@ebc635de3104104eaf31d6f3a58c58c9a694a655) that do not use Google Test, but instead use our `CHECK` macros to exercise the code.  There is the caveat that these macros compile into no-ops when built for release mode.  If not pulling in Google Test is preferable, let me know and I can update the pull request.