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 ... wondering if we need to say anything about implementation defined behaviour.

The issue should remain open as we still need to amend the XProc schemas to allow foreign element wherever a p:inline can exist ..  so we keep the issue open.