Use ADS123X library by Hamid Saffari

[amotl]

Through [1], we found the ADS123X library [2] by @HamidSaffari to be the most canonical one these days. It builds upon the HX711 library [3] by @bodge and the ADS1232 [4] by @jeffkub.

Whether this will compile on Arduino Due (#18) and on Arduino MKR1000 (#20) is a different story, but nevertheless we should add the canonical driver to the repository on order to be able to build upon it.

[1] https://community.hiveeyes.org/t/code-fur-ads1232-oder-ads1234/392/13
[2] https://github.com/HamidSaffari/ADS123X
[3] https://github.com/bogde/HX711
[4] https://github.com/jeffkub/beer-gauge/tree/master/firmware/beer-gauge