Plugin: user service - add new request type: grouplist & usergrouplist which returns xml with groupnames elements containing all group names or group names of a user.
Fix 'RFC 6121 8.5.1.  No Such User' for IQ stanzas

IQ stanzas, which were routed to a non-existent user were falsely replied to with an IQ-result (e.g. ping).
This fix checks for the existence of the user and returns an error, in case the user does not exist.
OF-397 Do not deliver offline messages to clients with negative priority

As per RFC 6121 8.5.2.1.1. Message and XEP-0160
OF-818 Message routing to bare JID can route to negative priority resources

This commit fixes the regression, that was introduced with the addition of Message Carbons.
Message Carbons are delivered to all non-negative sessions.
If there's no Message Carbons-enabled session, it will be delivered to the highest priority session only.
Merge pull request #38 from sco0ter/of810

Fix 'RFC 6121 8.5.1. No Such User' for IQ stanzas
Merge pull request #40 from sco0ter/of818

OF-818 Message routing to bare JID can route to negative priority resour...
Merge pull request #39 from sco0ter/of397

OF-397 Do not deliver offline messages to clients with negative priority
Merge pull request #21 from meneo/master

Plugin: user service - add new request type: grouplist & usergrouplist
Correct the target handler in MUC Admin UI (which should be IQAdminHandler)

(instead of IQOwnerHandler)