New field types and decoders (C & Javascript)

[xoseperez]

New field types (using IPSO codes):

• generic sensor
• voltage
• current
• frequency
• percentage
• altitude
• power
• distance
• energy
• direction
• switch

New features:

• New methods: getTypeName, getError
• Included C decoder with JSON output
• Included JavaScript decoder compatible with TTN and NodeRED
• Unit tests with AUnit (tested on Arduino Leonardo, M0Pro and Wemos D1)

The library is backwards compatible with the previous version, but it has been reworked to abstract several methods and avoid duplicate code. All the new features are being unit tested using AUnit. The README.md file has been updated to show the changes (the API.md file has been merged into the README.md file).