first cut at issue #38 allow p:inline to be optional

(pls ignore the flapping about with depify user committing, my git global user.name went nuts)

This is an attempt to progress issue #38.

The issue should remain open as we still need to amend the XProc schemas to allow foreign element wherever a p:inline can exist as well as amending the changelog.
the sentence XProc own namespaces is confusing
you should explicitely tell which namespaces are concerned (because http://www.w3.org/TR/xproc/#namespaces defines c: which seems to not be a problem in your example