Watch configuration directory for changes

[cwhitney]

Reported by Andrea Aime:
"Having to restart the GeoWebCache each time the configuration changes is a little cumbersome. In GeoServer we do have active listening of file changes for a few property files, like the users one. Basically we look at the last modification date of a file and reload it if necessary. Listening to a new file can be done by looking at last modification date in the configuration directory."

[arneke]

Moving to 1.1, it really requires som input from the user as well, and reloading isn't that big a deal.