Check for a tiddlyweb mode when pushing tiddlers

If set to true, tsapp doesn't expect a tiddler to go to a public or private bag, as dictated by TiddlySpace.
Support a server prefix in the URL for GET, PUT and DELETE

If `server_prefix` is set, this is prepended to the URL when looking up tiddlers, sending PUTs and DELETEs.
Merge pull request #12 from pads/twapp

Config modifications to suit working with a TiddlyWeb server