Xbee

* Implemented threading support.
* If a callback method is given to the XBeeBase constructor, a new thread will automatically be spawned. This thread will read from the serial port and call the given callback method whenever a valid data packet arrives.
* XBeeBase.halt() was added. This method ensures the proper shutdown of a separate thread if one has been spawned. This method must be called before closing the serial port used by an XBee instance in order to prevent improper use of the serial port.

* Bug fix: Now supports receiving I/O data with 64-bit addressing.
* Previously, an exception was raised when a packet with ID 0x82 arrived, which contains I/O samples with a 64-bit source address. This has been fixed.

* Now supports both Series 1 and Series 2 modules (the API turned out to be the same).
* API frame logic was split into its own class, APIFrame.
* XBee renamed to XBeeBase.
* XBee1 renamed to XBee.
* Tests updated to reflect changes; API frame tests moved to test_frame.py, now test APIFrame instead of XBee base class.
* Test files renamed appropriately.
* PyLint score improved.
* Various docstring updates.
* Updated example code to reflect changes.

* Initial Packaging.
* Fully restructured into a unified API with tests.