Library shelves implemented

The new action, -library, opens an inventory stocked with the books in the library. Currently limited to 54 slots because the client starts rendering it oddly at that point.

Other changes:

• Implemented a -save action, useful when autosave is off.
• Added a missing permissions to plugin.yml (.action.prepend).
• Changed library storage to TreeMap rather than HashMap.
• Re-creating the FileConfiguration on each save/load to be sure it's a fresh one.

On the TreeMap thing... I realize that TreeMap is way way slower than HashMap by a factor of ... well, a lot.
The thing is, that a TreeMap will automagically alphabetize the library, so I don't have to do that myself, and I can't think of a case where a library would have so many thousand books that you could actually notice the difference.

The -reload action doesn't *actually* reload the configuration.  
Fix is in the works, so stay tuned!