Started prep for LINEAR nav mode...

* Added methods to TestSessionController to handle 'next' selection
* Preparatory refactoring of engine classes to accommodate this.
Fix to 'may review item' logic
Continued test navigation logic...

* Merged original linear logic into later logic
* Started support for multiple testParts
Added some linear examples & renamed existing
Minor tidy of rendering XSLT
Continued linear navigation - partly working now
Fix to default value for maxChoices in 5 i'actions

I discovered that the default value for maxChoices changed from 1 to 0
somewhere between drafts 2 and 13 of the QTI info model for
choiceInteraction, hotspotInteraction, hottextInteraction,
positionObjectInteraction and selectPointInteraction.

These were very poorly described in the changes document. Chances are
there will be similar issues elsewhere
Added samples for validateResponses/allowSkipping
allowSkipping/validateResponses now as spec

* Works reasonably for both linear & nonlinear navigation mode.
* Now only applies to individual sub mode, as stated in spec.
* Updated samples to test this successfully
Fixed regression added in 3fdd471
Minor simplification to item/test delivery svc
Removed CandidateEventCategory
Further tidy to cand item/test service beans
Updated JavaDoc to reflect current state
Minor update to test samples