Remove depMap.
External events.
Add support for external events.

Something is off with DPOR.  Need to make tests for it.
TransDPOR checkpoint.
Switch to traditional DPOR.
Change the way alreadyExplored is being tracked.
Change the way explored interleavings are managed.
Change how the next trace is scheduled.
Cleanup.
New lines.
Merge pull request #22 from NetSys/dpor

Add external events to DPOR.
Merge commit '4d382e5d805eeeca897fb6d9cbf09b1cadc80df0' into colin-rbcast