Preserve timer value in the X resource DB

This is an initial implementation for preserving the timer value in the
X resource database. Based on the proposal from @blueyed in #8
Keep the state only on awesome restart

This doesn't spawn a new process every second but attaches to the
awesome exit signal. #8
Enable the keeping state logic by default
Merge pull request #10 from nikolavp/xproperty-preserving

Xproperty preserving