Add initial version of POPWRITE

These sources were copied from my earlier Retrochallenge 2013 Summer
Challenge entry where it was assembled using the Applesoft BASIC based
assembler that I wrote for that challenge.

This version is buildable with snap and produces the same output using
both assemblers.  I had to add an ORG directive and change ` ASC string
delimeters to "
Correct location of extra 8 cycles in self-sync bytes

The code in the TRACKWR function of POPWRITE was incorrectly prepending
8 cycles of delay to self-sync bytes instead of appending the delay.
I think this was the cause of the occassional read failures that I
would see in PoP during bootup.
Change POPWRITE ORG to $2000

This is more consistent with other samples.  Previously I was using
$9100 but that was only because the AppleSoft based assembler that I
wrote for the last Retrochallenge forced the use of this origin
address.
Refactor POPWRITE to not use 65c02 instructions

I want to increase the Apple models on which this code will run.  I
started by getting rid of the XC directive which enabled 65c02
instructions in the snap assembler.  Then just fixed the resulting
errors.
Add photo of nibble timing