(#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.
Typically instead of doing explicit regex matching like this, you should use the express route templates.  I assume Express optimizes that; otherwise every request has to pass through this.