Added documentation, removed library from Makefile

Making the mqtt as a .a library is a project for a later date.
The recipe was broken and so was removed to avoid confusion.

Added the Mozilla License header to the new files. Also a few
small corrections and additions to the Doxygen documentation.

Signed-off-by: A.M. Rowsell <amrowsell@frozenelectronics.ca>

mqtt.h

@@ -100,7 +100,7 @@ void ICACHE_FLASH_ATTR reconnected_callback(void *arg, sint8 err);
 /**
  * A callback function which is called when TCP connection is successful.
  * @param *arg A pointer to void, which needs to be cast to espconn *pConn
- * @returns Void
+ * @return Void
  *
  * This function is called once the TCP connection to the server is completed. This allows us to register the callbacks for received and sent data, as well as enable TCP keepalive and set validConnection in mqtt_session_t to 1 to show the connection has been successful.
  */