(#75) - Attachment support for design docs.

Fixes #75. Style sheets now load in the couchapp I'm testing with. Top is CouchDB, bottom is pouchdb-server:

![couchapp](https://cloud.githubusercontent.com/assets/1750170/4149522/dfc4cfa6-3434-11e4-819b-9ef948b83a79.png)

The difference between the images above is caused by Futon (and some of its .js files commonly used in couchapps) not being present. But since the same problem will occur in couchdb 2.0 too, that's probably not worth fixing.