Added AMQP predicate documentation.
Renamed mq.scm to amqp.scm so that it describes better for which test case it is.

--HG--
rename : test/tests/net/mq.scm => test/tests/net/amqp.scm
Start supporting MQTT.
Implemented CONNECT and DISCONNECT message for client.
Implemented subscribe and receiving message.
Implemented client APIs
Added mqtt-ping (PINGREQ)