Added test case and bug fixes for basic RDFa parsing; improve coverage of ODRS terms

I wanted to ensure that data kitten extracted all of the terms from the ODRS vocabulary so potentially more of this could be used in the certificates or other consuming apps.

The changes include:

* test case for the RDFa parsing
* bug fix for the update_frequency method (was using wrong namespace)
* Switch to using RDF::DC built-in vocabulary rather than dct() method
* addition of first_value helper and refactoring so that individual missing properties don't cause failure to parse all distributions, rights, licenses, etc. I've retained the exception handling as a fall-back
* extended the Rights object to include more terms and test case to check these are extracted

(Note: I've done this in a separate parallel branch to the Linked Data changes, so depending which one is accepted first, I may need to resolve some merge conflicts before the other branch can be merged down).