give enable/disable interrupt instructions something to do when called instead of a nop
check for interrupts
begin timer class
constants to define the addresses for each register
define frequencies
class to encapsulate counters
structure of timer
timer#read
timer#get_frequency
timer#write
connect timer to mmu
Create timer to throw periodic interrupts.

Closes #28
tie timer into rest of emulator
Timer and scaffolding for interrupt handler.