Rewrite Open Trickler w/ Python

[erichiggins]

The primary goals of this PR are to improve stability, maintainability, and code readability.

Here a few, more detailed, reasons:

• A classic infinite loop w/ synchronous calls is more simple than event-based, asynchronous code
• Python is easier for beginners and non-programmers than NodeJS
• The NodeJS binary installed through Buildroot creates significant startup latency (up to 30 seconds)
• Bluetooth libraries for Python have matured since the first Open Trickler was created, removing an obstacle

Other functional changes in this PR:

• Added PID controller (user configurable & tunable) for improved trickler performance
  Issues: Improve trickler performance (speed, accuracy) #15
• Uses PWM motor control for smoother vibration motor operation.
  Issues: PWM control for Vibro Motor #33
• Memcached is leveraged to separate concerns (e.g. main loop, motor control, scale control, bluetooth) and enable expansion (e.g. web-based UI)
  Issues: Web interface for Trickler #20
• Separate scale control (trickler/scales.py) was written to support multiple scale models, including command-line testing
  Issues: Use other RS232 scales i.e. Acculab #35 Connect to mettler toledo #39
• More stable Bluetooth advertising and connections (not blocked by scale problems, etc)
  Issues: Mobile app does not update target weight when BT connection is re-established #18
• Added support for servo/stepper motors (for things like powder drops or powder augers)
  Issues: Add powder-drop support #21

Open issues:

• Verify full compatibility w/ existing mobile apps
• Create alternate bluetooth module for future mobile app rewrite (e.g. don't use zero 0 values where they can be confused with null values)
• Control Pi's onboard LED to indicate trickler status. Also support alternate/external status lights, as mentioned in Start Up Log/Status Indicator #30
• Create web UI (see opentrickler_api.yaml)
• Add support documentation to the webpage hosted on the Pi. Include details like the IP address, wiring guide, what the lights mean, debug tips, FAQ, and how to get more help.

[erichiggins]

SD images seem to work as expected. Merging for now to prioritize one language and major version over two. Any other bugs can be fixed as needed.