DS-1990: Events should contain all identifiers, not only handles.
Fixes typo in a javadoc comment.
Bitstreams should set identfiers to Events as well.
Enhancing the PR for DS-1990.

Adds a cache to the Identifier array in the DSpaceObject class.
Renames the method from lookupIdentifiers into getIdentifiers.
Asynchronous IdentifierProvider should call DSO.resetIdentifierCache.

Adds a note to the javadoc of DOIIdentifierProvider to reset the
identifiers cache in case of asynchronous identifier registration.
Adds a call to reset identifiers cache to DOIIdentifierProvider which
uses asynchronous registration.
DSpaceObject.getIdentifier(Context) should never return null.

If an DSO does not have any identifiers (including handle) this method
should return an empty string array. G.e. an item during submission
(workspace item) has no identifiers at all. This commits prevents NPE in
event handling classes.
Merge pull request #537 from tuub/DS-1990

DS-1990: Events should contain all identifiers, not only handles.