ACT-1963 Data Object support for Activiti Modeler

Removed the joda dependency for XML and JSON conversion and used default
SimpleDateFormat similar to the runtime due date values.

Updated the xml conversion code and tests to use the same formats
simplified the test comparison (got rid of Calendar code).

Fixed NPE when data objects have no value - similar issue to ACT-2005
for Designer code.

Change Id: 6b6768fea020d4bf6907866652f4262f8bb0fee1
Merge branch 'act-1963' of https://github.com/lsmall/Activiti
Small changes