Handle removing the unfired triggers that where added as part of a list when one fires
Issue #16: Skeleton for deregister_callback
Issue #16: Derive ClockCycles and RisingEdge from Edge to inherit unprime method
Issue #16: Convert the world to have one handle type to the vpi world, clock still to do
Issue #16: Move cleanup of callbacks to destruction not after the callback has happend
Issue #16: Implement unprime from the scheduler
Improve class hiearachy for triggers
cleanup
change clock driver to use common sim handle from Issue #16
Issue #16: More fix ups to deregister
Merge branch 'master' of github.com:potentialventures/cocotb