Revert "Removed SequenceGenerator because it does not work in OpenJPA"

This reverts commit f83f36d8663772e5b102cb6004c59679380239c0.
FIX: Type "org.jbpm.task.impl.model.EmailNotificationImpl" uses a flat
inheritance mapping, but declares a table name of "EmailNotification",
which does not match the superclass table "Notification".
FIX: You have supplied columns for ... but this mapping cannot have
columns in this context.
FIX: No metadata was found for type "class
org.jbpm.task.impl.model.DeadlinesImpl"
FIX: (more) You have supplied columns for ... but this mapping cannot
have
columns in this context.
Replace "in elements" with "member of" in named queries
Replaced Collections.emptyList() with new ArrayList() because OpenJPA
does not know how to handle Collections.emptyList()
Adding missing class to persistence.xml
Use merge instead of persist because OpenJPA does not automagically
merge new Object()
Adding tasks does not update the id. We have to use the id returned by
the method.