add power-of function to simple-calc

also had to edit Makefile to link Math library (-lm).
+ 1 or 2 'fixes' (use num_options somewhere over a static number)
refactor: do/while flag, check input somewhat, add default case

TODO: check input is an integer
add TODO.md
Merge branch 'master' of https://github.com/raehik/aqa-computing