TODO: Add utility routines for vectors

[dj-on-github]

The 6502 has vector locations for reset, NMI and BRK/IRQ.

Let's add utility routines to read and write the vector locations.
asm6502.setvector_reset(self, address)
asm6502.setvector_nmi(self, address)
asm6502.setvector_irq(self, address)

asm6502.getvector_reset(self) --> int
asm6502.getvector_nmi(self) --> int
asm6502.getvector_irq(self) --> int