BMS state of charge tracking

[colescott]

The state of charge (or I guess discharge) should be tracked by watt-hour counting from the current sensor and stored in the internal memory of the LPC1768.

Tasks:

• Implement analog in for current sensor and validate
• Add KVStore from mbed to store configuration and SoC values
• Compute discharge rate and update SoC accordingly
• Publish SoC to CAN bus
• Reset SoC when charging (this might require smarter charging logic)
• Add a fault for low SoC