Add URLConstraint

Currently to check that a document field contains a well-formed URL, people are using a xs:pattern restriction (implemented by PatternConstraint) with a huuuuuge unreadable regexp.

We should instead provide a custom nxs:url constraint that is checked server-side using the URI class. This will make things much more maintainable (and we could have various parameter flags that say for instance http-only, no query parameters, no fragment, etc.).