adds base url parameter to url_builder_factory, closes #7
Merge branch 'master' into fix/2
instanceSpec: fixes relation "no private key" test

@blackjid, the original test expected $url() to be null, even though
the api-returned object had a valid id and context. When a resource
does not have a base url and is loaded from a context (relation), the
context url is used as base url.

I also think that this kind of tests belong in a url builder exclusive
spec.