Add ATtiny 25/45/85 support

Add ATtiny 25/45/85 experimental support.

https://github.com/damellis/attiny and https://github.com/SpenceKonde/ATTinyCore cores

Experimental SAMD51 and move docs and extras to other repository

• Try to fix SAMD51 boards; still experimental as I have none of this boards.
• Move docs and extras to sparse repository: https://github.com/Naguissa/uTimerLib_doc_and_extras
• Version bump to 1.5.0

Break different ARCHes on different files

Now each architecture has its own file for ARCH-dependant functions. Easier to read and manage.

Add ST's Arduino Core STM32 support

This adds support to ST's Arduino Core STM32, as requested by @AndKe in #9

• STM32 previously supported core (still supported): https://github.com/rogerclarkmelbourne/Arduino_STM32
• New STM32 Core support: https://github.com/stm32duino/Arduino_Core_STM32

Fix STM32 seconds interrupt handling

Fix STM32 seconds interrupt handling, reported by @mignon-ne

Fix function return type for setTimeout ones

Also add Travis-CI status badge

Added AVR 32U4 support and doxygen

• Added AVR 32U4 support (Timer3)
• Updated in-code documentation
• Added Doxygen HTML files, available on doc/index.html

AVR timing calculation fix

AVR timer calculation fix, #6

SAMD21 timing fix

SAMD21 timing fix by @svlPanelMaker - #5

Add ESP32 compatibility

Uses same system as ESP8266