Fix a typo on uTimerLib_setTimeout_us_example_serial example

Fix STM32 - solves #23
Improve serial examples

Trying to fix issue #24

Fixed imprecisse timing on SAMD21 (Arduino Zero). This solves #21. Thanks to @daniel-mohr

Fix ESP32 typo

Add compatibility with new STM32 core versions. Fixes #16 and #17

Note: Still maintains retrocompatibility with older cores.

Fix a bug when using multiple timers (continuous, not at same time) on ESP32.

Fix clearTimer functionality on ESP32. Fixes #14

Enhance ESP32 support using OS hardware timer

Add DigiSpark AVR support using Timer0 (Timer1 is in use for milis)