Ensure thread safety

[amorphic]

If thread A and thread B both send commands to a tosr0x device in quick succession it it currently possible for thread A to receive thread B's response and vice versa.

Currently there is thread locking in place which attempts to isolate each thread's communication with the device. But apparently this isn't working in all cases.

A good suggestion in this post suggests using a queuing mechanism:

http://stackoverflow.com/questions/4043193/python-how-to-share-a-serial-port-with-two-different-threads-class-a-class-b