fix the problem of addition with next_topic in emqtt_topic_new function
handle the error suback and pass information for the user
in EMqtt.h: add subscribe function and change name of callback error subscribe function.

in emqtt_client.c: change test on topic_name, change the msg of subscribe and add callback
on subscriber

in emqtt_sn_sub.c: implement functionnality of subscribe and suback error
change the information in printf in _topic_received_cb
in EMqtt.h: add connection states and macros for the connection process

in emqtt_private.h: add variables and callback function for the connection process

in emqtt_client.c: add timeout timer for retry connection, handle the connection state in
_mqtt_sn_connack_msg, handle the connection state in emqtt_sn_client_connect_send

in emqtt_sn_sub.c: implement new functionnality of connection process
Merge pull request #3 from JulienMasson/master

Modify functionnality of connect, subscribe and suback