Added NIST clock and generating HTTP header Date

Application uses reading network date and time from NIST time servers for regular updating internal timer in order to keep correct system calendar date and time. It substitues RTC (real time clock) hardware and allows displaying real starting time and current system time as well as generating correct HTTP header Date in web server responses.

Reduced version for ESP8266-01

Some programming content and functionality has been removed and sacrified in order to manage uploading all scripts in compiled form to the ESP8266-01 due to its small free space on its file system.

• Removed comments and not necessary debug printouts from init.lua.
• Removed generating HTTP header Date, hence removed names of week days and months from init.lua.

See README.md for instructions how to compile and upload scripts to ESP8266-01

Elapsed time in AT format in the HTML page footer.

• The system calculates uptime time (since recent restart) in AT time format, i.e., days, hours, minutes, seconds, e.g. 2d 3h 4m 5s with omitting a part with zero value.
• Fixed typo at generating HTTP header 501.