(#14) - adds rewrites support

Requires https://github.com/pouchdb/pouchdb/pull/2360 to be merged first. I'm new to Express (& pretty much node.js in general), so I hope this is done 'the right way'. Anyway, feedback welcome.
You mean pass in as first argument to app.use? I tried that, but then req.url becomes relative to that making it impossible to rewrite a url to anything lower than /db/_design/name/_rewrite, which is useless. I tried using '..' too, but it seems express doesn't normalize that away. A regex was the only solution I could come up with (although as I said, my Express knowledge is limited.)