Technical Cleanup:

* Setting up the frequency (and thus enabling all timers)
  whenever the library is used is selfish:
  Maybe someone else wants to use one or more of the
  Timers/Counters in his code for other purposes.
* We don't use the register RA and its triggered output line
  TIOA in any way, so we don't need to care to set it up.
fixing bug: check in start() if _frequency is > 0.
bug fix for 876a136  -  thx @ivanseidel
Merge pull request #14 from pklaus/technical-cleanup

Technical Cleanup