Override getValue() in OriginalMetadataAnnotation

This prevents getValue() from returning null, as it previously delegated
to the implementation in XMLAnnotation, which returns XMLAnnotation's
internal 'value'.

Noticed by Stéphane Dallongeville.
Fix original metadata retrieval

Without this change, calls to getOriginalMetadata(...) will return a
Hashtable of size 0 when the XMLAnnotations are instances of
OriginalMetadataAnnotation.
Add test class for the OMEXMLService
Merge pull request #629 from melissalinkert/rebased/develop/originalmetadata-value

Override getValue() in OriginalMetadataAnnotation (rebased onto develop)