Replace incorrect object reference.
Check for object existence before dereference.
Enhancements to relationship management.

Allow addRelationship() and editRelationship() to handle multiple interstitials of the same type
(e.g. multiple "identifiedBy" values for an object_x_vocabulary_term relationship).
Add wamDataImporterPlugin.

+ Allow values to be split using configurable delimiters, to create multiple interstitials from a single source value.
+ Allow values to be transformed into IDs using DataMigrationUtils::getEntityID(), using the "_translations" sub-key
   under "interstitial" in mapping configuration.
+ Support other transformations / lookups in future if required.
Merge pull request #83 from kehh/importPlugin

Import plugin implementation