Update usage message
Move sched impl to cpp file
Move tick freq. responsibility into Watch class

Watch class uses last tick timestamp to determine whether a second
have passed already. Watch::frame() is invoked at 30 hz, freq.
const in Sched.
Start using Boost.Chrono for time calculations.
Make Watch tick independent of Sched cycle freq.

Watch will compensate tick() invocation count if more time passed
in one Sched cycle (that is, if tick() has not been invoked for
more than a second)