Update isa_spec.md

Added halt and debug
Implemented the reg_shift module - needs verifying

resolves #14
 On branch develop
 Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)

	modified:   src/rtl/mux_4to1.vhd
	modified:   src/rtl/reg_shift.vhd
Merge branch 'develop' of github.com:willprice/vhdl-computer into develop