If ProgramCounter reaches the end of memory, loop.

Currently, if the Program Counter is going to breach 11111111 then it's set to 00000000 - which makes it impossible to ever set it back to the first instruction (00000001).  This fixes it so that it loops correctly.
Merge pull request #8 from andrewducker/ProgramCounterLoops

If ProgramCounter reaches the end of memory, loop.