Matching standard Arduino cores, make the default analogWrite() take
values from 0...255.  Users can always use the analogWriteRange() call
to change to a different setup.

Fixes esp8266#2895

Add a `analogWriteResolution` which takes a number of bits and sets
the range from 0...(1<<bits)-1

Remove the PWMRANGE define.  It's non-standard and not generally valid
(i.e. it's fixed at 1024 of 256, but the real range varies depending on
what you last set).

Also add note about the change and how to fix pre 3.0 applications.