base_url behaviour change

I've included some extra tests to show the behaviour of the base_url.

Now that I'm writting this, documentation changes to explain it will be
needed.
Documentation about asset specifications.
whitespace
Resolve sources for static url as spec

When the user registers a static url using an asset spec,
`resolve_source_to_url` must fall back to calling `static_url` with
the original item, in case it is a spec with a matching prefix.
Handle asset spec base url with relative output

When the base url has been manually configured with an asset spec,
handle the case when the static route has been registered with an
asset spec as well. In this case, the base class resolver won't
properly prefix the url so it needs to manually be based off the
configured value. Either this derived path or the base class result
must then be resolved with `static_url` to finalize the result.