Replace print statements with python standard library logging

[ThomasDavid0]

This is so the std output can be controlled by code that depends on pymavlink.

[MauroPfister]

I'd love to see this changed at some point. Thanks for taking the initiative @ThomasDavid0!

One small remark: AFAIK the recommended way to write log messages is logger.info("timeout setting %s to %f", name, numeric_value) instead of logger.info("timeout setting %s to %f" % (name, numeric_value)).