Corrected channel config

[jjderooy]

This branch fixes the configuration issues with the labjack channels.

• Differential channels are now sampling correctly.
• Sampling system has been streamlined into a single well-documented file.
• Channel modes can be set to SING or DIFF, and with 1X, 10X, 100X or 1000X gain by OR-ing preconfigured bytes together.
• Collected data is json dumped to a log file, and the most recent data point to a tmp file for reading by separate websocket process.
• Gains and offsets are correctly set for all sensors except thermocouples. A separate branch has been created for this feature.

Read speeds are very fast from the LJ, with sample rates of up to about 20KHz achieved.