ESP32 #78: MQ Gas sensors

Testing the MQ Gas sensors with the #ESP32.

Arduino Example
https://github.com/pcbreflux/espressi...

More Info
https://playground.arduino.cc/Main/MQ...

Development Environment
ubuntu 17.10 64 Bit (https://www.ubuntu.com/) (unity)
Eclipse Oxygen (https://www.eclipse.org/)
esp-idf commit 475ffe78e8c7674a6b9b2dff78774a4a8b66470a Fri Feb 2 19:03:20 2018 +0800 (https://github.com/espressif/esp-idf)
xtensa-esp32-elf-gcc (crosstool-NG crosstool-ng-1.22.0-80-g6c4433a) 5.2.0 (https://dl.espressif.com/dl/xtensa-es...)
Arduino 1.8.4 (https://www.arduino.cc/)
arduino-esp32 commit a153f6ad280e5932be9c80341556348ff505d586 Thu Feb 8 15:56:13 2018 +0200 (https://github.com/espressif/arduino-...)
KiCad Nightly-build (http://kicad-pcb.org/)