Simple leveldb store.

Oddly the levelup api looks pretty much like the pagehandler api, so all I had to do was check for pages that don't exist on get and fall back to the fs store. This lets us get plugin and default-data pages without having to replicate that logic. It has the interesting side effect of putting a wiki used with the file system store into read only mode, such that old pages can be read, until they are modified and the db copy shadows them. I kind of like it, but if we don't want that we can refactor out the static page handling into it's own thing, and require it with each new store.