Use relative-root paths instead of relative paths

Code in db accessing the database performs relative paths:

    url: this.url + '/_design/' + exports.encode(name) + '/_rewrite' + path

but this assumes the code is being called from the root path, which is not always the case.  I notice the comment that:

     397 // TODO: handle full urls, not just db names
     398 exports.use = function (url) {

but perhaps this is not directly related to this point.  

I believe this can be handled either by demanding that 'url' has a forward slash (probably nicer) or check this in exports.request .