Fix Timer rollover problem

Add and subtract: good.
Compare: bad!
Changed to a subtraction yielding a signed result.; relative math works
across rollovers.

Also modified the BenchmarkRTCTimer example to include a rollover test,
but this requires either (1) making RTC::s_uticks visible temporarily
(for the build), or (2) adding a `micros( new_value )` mutator to RTC.
I think I'd suggest the latter.
Merge pull request #263 from SlashDevin/Timer

Fix Issue mikaelpatel/Cosa#262
Merge branch 'master' of https://github.com/mikaelpatel/Cosa into add-adafruit-boards