WIP: Scheduled service - Create plugins that run scheduled tasks on behalf of the router.

This enables plugin authors and users to allow I2P to use a ClientApp to schedule the running of an external process at regular or semirandom intervals. The idea is to use it to periodically fetch git sources and run a static site generator like Jekyll or Hugo to generate pages for the eepsite/docroot directory, effectively turning the built in site into a clone of github pages for in-I2P sites.

It could be easily adapted to many other types of tasks, like periodically fetching and aggregating RSS feeds or generating torrents and placing them in the i2psnark directory. I detailed the working of the class in the javadoc.

Re: WIP it's actually largely implemented, but I did most of it this evening and it certainly bears some testing and a review of the concept.