Organize the package structure for testability and portability

[esimkowitz]

• Prime the serial connection by running set_all_leds first, still don't know why this works but it's reliable enough. It fails the first time it runs, still working out why.
• Move the Sphero SDK into a sub-folder of the rvr_node package that depends on it.
• Structure the setup.py to load all the required packages and copy the SDK source into the rvr_node package.
• Add a new interface that abstracts the SDK functionality and adds the ability to mock the SDK for testing on machines that cannot open a connection to the RVR.
• Improve the helper script for launching docker compose, adding flag arguments for building the container image and for mocking the SDK.
• Improve the Docker image build to minimize steps that require rebuilding