Support offline messages according to XEP-0160
Add offline message limit
Disable offline message support by setting limit to zero
Fixed counting per user
Add msgoffline iq to disco_info
Skip when addressed to an offline user
Fixed ready_test; do not override from variable
Offline msg limit is related to the recipient
Added sql tests for offline message support

* Missing receive and response tests
Referring to XEP-0160 if recipient becomes available
Deliver offline messages to all non-negative priority's

If all of the available resources have a negative presence
priority then the server SHOULD either:

a) store the message offline for later delivery or
b) return a stanza error to the sender, which SHOULD be <service-unavailable/>.
Add destroy_message method to sql adapter
Do not use sql related stuff outside of the adapter
Add offline message handling to the adapter connection
Fixed correct return value for limit test
Merge pull request #30 from Zauberstuhl/support_offline_messages

Support offline messages