Python decoder. Added array support.

Now generated python code can successfully decode message containing
array(s) of standard C type. One drawback - only char[] fields decode as
python string, but uint8_t[] decode as python list. As I know there are no
mixing of uint8_t/char in common.xml so it is not a problem. Pull
request also contain tests for array handling. I can not find where
to place pythonic tests so I have placed them in pymavlink/tools dir.