fix(lock): fix bug and improve semaphore test coverage

Calling Unlock() on a Lock that did not hold the semaphore would panic
when the semaphore had >0 (other) holders.
Added test coverage for all Semaphore actions.
Merge pull request #7 from jonboulle/tests

fix(lock): fix bug and improve semaphore test coverage
Merge branch 'master' of github.com:philips/focaccia