Added apu.go file which defines APU and contains the RP2A03's APU
registers.  Implements the Memory interface.

Added memory.go file which defines MappedMemory which allows mapping
anything implementing the Memory interface over specific addresses of
another Memory instance.

Added cpu.go file which defines RP2A03 and contains an APU and 6502 CPU
from the m65go2 package.  Takes a MappedMemory instance and uses it to
map the proper addresses into the APU.