Better support for SP contacts

[mauromol]

This is an enhancement request to better support contact declaration for SP metadata.
The Italian SPID system, for instance, requires the Service Provider to declare a ContactPerson with contactType="other", a Company name and possibly a phone number, all features currently not supported by java-saml (which provides the ability to define just a technical and a support contact with minimal information). (*)

I may provide a PR to support this: you can preview the changes at: mauromol@a999053
Before providing an actual PR, though, I must wait for your feedback on previous PRs, because this change builds on top of other work like #307 and #315.

(*) Just for completeness: the SPID system also requires the inclusion of a <md:Extensions> element on the other ContactPersons: however, I think this is too much SPID-specific and atypical to be supported directly in java-saml. Nevertheless, the extension mechanism provided by #321 should make it trivial to also satisfy such a special requirement by library consumers.