add the Ophis assembler

In the third post at http://forum.6502.org/viewtopic.php?f=2&t=2497 (in the topic "6502 HLL call conventions"), rwiker writes,
> David A. Wheeler has some thoughts about 6502 code generation, that may be relevant to this discusssion: http://www.dwheeler.com/6502/

On that page, David A. Wheeler writes,
> 6502.org lists lots of assemblers. One they've omitted (why?) is Ophis, a 6502 assembler written in Python in 2006-2007; looks nice, though its syntax is a little different.

Hence the recommendation that we add it.