Trigger one file each trigger interval

Use an interval timer to trigger each watched file in turn. This ensures that if there are any missed changes because of inotify, those files will eventually be synced. Trigger interval can be used to tune the load on the system because of these operations.
Make periodic trigger interval configurable
Add periodic trigger interval to config file
Update configuration example in README
Merge pull request #22 from poplatek/feature/periodic-trigger

Periodically trigger each file to catch missed changes