Updates to the Tellstick binding

Pull request includes new feature and a few bug fixes:
- configurable resend interval per device (was always 250ms)
- fixed rounding errors, sending ie 'dim to level 10' reported back 'updated to level 9'
- dim to 0 now sends tellstick command OFF, dim to 100 sends ON
- other minor changes

(commit dde9a26 and 73d1e6f are duplicates - my mistake)