use JSON Pointers only when given in accept header

Serialize JSON using pointers only when the client explicitely requests
them using accept header parameter "x-json-pointer=true".
allow cache of caching internalizers to be cleared

Clear caches before running tests.
change widget model to allow non-public properties

Distinguishing between public and non-public properties allows widgets
to model some of their internal state such that only precursor actions
can satisfy requirements of certain actions. Public properties requiring
a filled valuei, however, can be satisfied by any widget instance. This
makes it difficult to model an internal state solely based on public
properties, as precursor actions are only required to clear public
properties.

Modify Mashup Ontology to include property :hasPublicProperty .

An action's published properties are derived from its effects (filled
properties) and its widget's public properties.
Fix tests to account for this change.
fix implementation ambiguity

Fix method name not matching the method's result (method name implies
the negated value).
fix PlannerJob to report any exception as failure