Support 'halting'

If the instruction `ADD PC, -1` is executed, the CPU will enter a 'halted'
state where time warp is unrestricted. Interrupts will wake the CPU and
reduce time warp.
Resume previous time warp speed when halting after an interrupt of a previous halt
Merge pull request #20 from dbent/topic/halting

Halting
Merge pull request #21 from dbent/version/0.3.0

Version/0.3.0