Flesh out the rest of the store methods.

Lots of boilderplate here but couldn't be assed to be clever about
it. It's at least explicit.

Note that when a bag is deleted we don't properly tidy up both sides,
we need to. When a tiddler is deleted we do.
Fix bag_delete to deal with bags in the binary store

Untested!
Test some binary images: kittens

Have also done some manual testing with tiddlywebwiki and
this store.