Added ability to control certain webapp features

I have added functonality to allow a QTIWorks instance deployer/manager
to decide whether to include the 'public' and REST features.

The defaults for both of these are now OFF (which is sensible), so will
need to be explicitly enabled if people want to use them.
Minor rationalisation of engine manager actions

* The bootstrap action no longer imports samples. (This change was made
as samples are now considered an optional feature, so arguably shouldn't
be loaded if they are not being used.)
* The reimportSamples action has been renamed importSamples
* The rebuildSchema action has been deleted, as it is now essentially
the same as bootstrap.
Minor code/comment improvements to service layer

* Renamed some ensure...() methods as assert...() when checking client
conditions
* Services now indicate whether they use authorisation or not.