moved BW::Reactor::PeriodicTimer from NSTimer to GCD Source.timer
moved BW::Reactor::Timer from NSTimer to GCD Source.timer
moved cancel to ensure that timer get terminated even if an Error occurs
it passes all tests related to reactor
removed the test case for \'common runloop modes\' since this option is not needed using  GCD
removed the test case for \'common runloop modes\' cancellation
added deprication msg for :common_modes in BW::Reactor::PeriodicTimer
Merge pull request #242 from seanlilmateus/dispatch_timer_from_nstimer_to_gcd_timer

Reactor timers from NSTimer to GCD Source.timer