Moved around some functionality and added comments

Made interrupts/init.c a little cleaner by moving most of the IDT setup
code to a new file, interrupts/interrupts.c. Kept the default interrupt
handler in init.c, but now it works with the PIC and APIC (sending
EOIs). A new file interrupts/defines.h was added to group common
functionliaty that might be required outside of the interrupts/ modules.
Also commented these function definitions in defines.h.
Making progress on the APIC.

Now the APIC is enabled and appears to work. To test, interrupts have to
be enabled by added an __asm__("sti") after apic_init() is called. Will
be added later.

Currently the APIC is being used in compatability mode with the old 8259
PIC. The next work will be to find and enable I/O APICs instead of the
8259.