Implemented a few util functions. Fix to the keyboard ISR.

ISR/Kernel:
*Removed the ISR routine from the kernel.c file. Works fine now.
*Still not sure from where the Protection exception comes from time to time.

Util:
*Created a basic directory layout for the utility libraries.
*The plan is for them to implement a part of the standard library, math functions and
basic string operations (maybe more in the future)
*Added the needed rules to the Makefile
*Implemented the strlen, pow and itoa functions (no testing conducted)