Reference the right namespace prefix for builtins.

This requires that we pass the schema object down from the resolver so
that we can give `Parser.parse_builtin_type` the correct namespace to
search under.

In service of this, we added a method to Schema to facilitate getting
the XMLSchema namespace.
Add XMLSchema namespace to fixture XSD in ParserSpec

This may mean that the library can no longer handle schema-less XSD
files, but I don't see how it could do that before, except in the case
where it happened to use the magic "xs:" prefix.
Fix specs
Whitespace!