More updates

Interrupts are now mostly working (QEMU has some issues, but they don't
seem to be strictly related to the code here, though it is being
investigated). There also were some build-system changes (Among other
things, there can now be multiple targets for building bootable images,
which are placed into the 'imgs' directory).
More various updates

Makefile stuff - pic8259 driver and timer now work among other things.
Update .gitignore settings