Implement mechanism for measuring time

[natiiix]

This will be useful for benchmarking various bits of code. The unit used for the measurement must be objective/consistent and it should also be sufficiently small. Using seconds or great units would require running snippets of code for a very long time to get any sort of accurate results.

This issue probably depends on #11.

See https://wiki.osdev.org/Programmable_Interval_Timer for possible information on this topic.