Refactored the codebase - make extensive use of ES6 feature like async/await

• Added: flag --run to upload command to run a file on NodeMCU diretly after uploading - feature requested on GitHub #19
• Added: return codes (0-success; 1-general_error; 127-lowlevel_error) - feature requested on GitHub #48
• Added: debug mode to show low-level error messages --debug flag
• Added: io-debug mode to show all rx/tx messages --io-debug flag (requires enabled debug mode)
• Added: experimental esp32 support - thanks to Spiritdude on GitHub #47
• Added: FAQ document
• Changed: cli arguments take presendence over config file - feature requested on GitHub #21
• Changed: the connector API has been splitted into multiple files
• Changed: added new cli logger
• Changed: requires Node.js >= 7.6
• Changed: moved images into assets/
• Removed: lua optimizer (replaced by luamin)