A threading class for handling the BT, sending, and receiving of EV3g
mailbox messages. Test script & EV3g program added.
Also altered the EV3Mailbox class to handle Python
types rather than its own.

This will handle the sending and receiving of mailbox messages in
parallel, just as the EV3g system allows.

It didn't make logical sense having the ability to override the decoded
data in the manner available. Only 3 char text should ever be
overridden
as a number, so have a force_number() method to re-decode the current
payload as a float.

Also does the name/value <-> payload conversion in the two class method
constructors.

Added a test client to show what's received from EV3g