Added a message encoding function that accepts a channel.

In the C-API, just like there are the `mavlink_msg_msgname_pack` and `mavlink_msg_msgname_pack_chan` functions, there should be  `mavlink_msg_msgname_encode` and `mavlink_msg_msgname_encode_chan` functions, as not all messages are sent over the MAVLINK_COMM_0 channel and need the appropriate sequence number set.
Merge pull request #121 from dimasad/msg_encode_chan

Added a message encoding function that accepts a channel.