Specify default base IRI

[bjdmeest]

Separate from #30 and #223, I believe a default base iri should be clarified in the spec : either that's the empty string by default (generating relative uris has been a use case that came up a couple of times), or something else reasonable (eg http://example.com)

[dachafra]

I can lead the actual work on changing the spec, but @chrdebru and @pmaria probably have an opinion on this before we proceed. Something to discuss in the next meeting