Issue 59: Support crontab ranges 'x-y' and 'x/y'

I started to refactor the `scheduler._range()` function a little bit and implemented support for `x/y` and `x-y` as requested in issue #59. The function was changed to resolve the `crontab` elements from complex to simple elements.

The following complex elements are supported now in `crontab` setting:

   * list, e.g. `1,2,3,4,8-10`
   * intervals, e.g. `*/2`
   * ranges, e.g. `0-8`

Since the resolving is implemented in a recusive way, it's also possible to combine all of them. E.g. you can use `1,2,3,4,8-10` or `1-4,5-23/2` or something similar. The first one will include the hours between 1-4 and 8-10, the second one will include the hours between 1-4 and each even hour between 5 and 23.