Decouple periodic timer creation from scheduling
Add a failing test
More options for periodic_timer

::BW::Reactor.add_periodic_timers takes an options hash.
options[:common_modes] allows one to request NSRunLoopCommonModes
modes for the timer.
options[:callback] allows one to supply an alternative to the block
options itself can be callable, which preserves the existing API
Test preservation of the existing API