add version routines and TIMEOUT_PUBLIC prefix to exported routines so applications can optionally compile completely statically (by defining TIMEOUT_PUBLIC to `static'

add invariant assertion code, and comment our source a little better (this really needs a formal description)

use timeouts_ prefix instead of timer_, which is in the POSIX namespace (they get all the good prefixes!

work backwards as well as forwards when calculating mask to overcome cummulative error when step is greater than period