(#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.
Ah okay, didn't realize that.  In that case it's great, but your regex can be optimized a bit to avoid backtracking:

```js
/\/([^\/]*?)\/_design\/([^\/]*?)\/_rewrite([^\/]*)/
```

:+1: after that.