modify CrashMonkey and BaseTestCase to make use of checkpoints

[ashmrtn]

Part of the revised version of #12.

Checkpoints require support across many parts of CrashMonkey. This part slightly modifies the way user tests are called so that checkpoint information is passed to user tests

For checkpoints, we can assume 2 things:

1. the user has just performed a sync/fsync request of some form
2. this call will block until all parts of the checkpoint are completed

The part of CrashMonkey that calls user tests should be modified to pass along the checkpoint number (generated by #41). BaseTestCase.h should be modified such that this is allowed.