Support for Digital sensors

- Sensor constructor accepts type property of "digital" or "analog", defaults to "analog"
- Sensor constructor aliases:
  - Analog (pass-through)
  - Digital  (wraps implicit type="digital" setup)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Updated grunt-contrib npm modules

- Updated grunt-contrib-jshint to 0.10.0
- Added esnext: true to allow for ES6 features
- Updated grunt-contrib-watch to 0.6.1
- Updated grunt-contrib-nodeunit to 0.3.3