Python callbacks: read, write, reti, int mode 2 vector

[ToniWestbrook]

Added:

• Python callback for memory reads (same format as input callback)
• Python callback for memory writes (same format as output callback)
• Python callback for RETI instruction. The added override to "on_reti" function always invokes base::on_reti, but also calls the on_reti Python handler if registered. The handler is called after the base function to ensure processor state reflects the RETI operation.
• Python callback for fetching the lower byte of interrupt mode 2 vector address. A "on_get_int_vector" function was added to one of the base classes (completely guessed which was the best class, please feel free to reorganize) with a default value of 0xFF to reflect was the previous default was. Then Python callback was added.

I tested these in the VM (which heavily uses all of this functionality) and it looks like it's stable. Thanks!

[kosarev]

Looks great, thanks!