(sramp-158) Auditing support in S-RAMP

In this pull request I have implemented automatic auditing of changes to artifact nodes.  Whenever an artifact is added or updated, an auditEntry will be created for that artifact.  An Atom API is provided to access (and add to) the audit trail for a given artifact or for a given user.

JIRA: https://issues.jboss.org/browse/SRAMP-157
JIRA: https://issues.jboss.org/browse/SRAMP-158
JIRA: https://issues.jboss.org/browse/SRAMP-159