fixed wrong implementation of mavlink parameter protocol
(http://code.google.com/p/arducopter/issues/detail?id=552)

libraries/GCS_MAVLink/GCS_MAVLink.*
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added function that stores an AP_Param into the 4 bytes of a float

ArduCopter/GCS_Mavlink.pde
--------------------------
corrected the code that receives/sends mavlink parameters in:

GCS_MAVLINK::handleMessage
	case MAVLINK_MSG_ID_PARAM_REQUEST_READ
	case MAVLINK_MSG_ID_PARAM_SET

GCS_MAVLINK::queued_param_send

TODO:
- APMRover2 & ArduPlane contain the same wrong implementation
- MissionPlanner uses the same wrong implementation and doesn't work
properly with the fixed code