Add support for python 3 for Arduino-ESP8266 2.6.x

This is a minor update which only adds functionality to newer ESP8266 Arduino cores, which use python 3.

Support for uploading to ESP8266 using esptool.py

Latest version of ESP8266 Arduino uses esptool.py for uploads. This release adds support for that.

Update plugin for newer Arduino IDE

• Travis updated to build against Arduino 1.6.12
• source updated to find the sketch folder
• dropped support for ESP31B

0.2.0

• Fix recursive upload (#5)
• Support for OTA upload, works only with the default port and no password (#4)

0.1.3

• Support for Arduino 1.6.6
• Support for new esp8266 tools layout (fixes esp8266/Arduino#991)