Added Math library

This should (for the most part) handle issue #6 for now, hopefully the implementation looks good. I've tried to mirror what you did in the other two libraries, also note that I've added a function in interpreter.c/interpreter.h to link a constant float to the namespace. One thing that I'm not sure (and maybe you have some input) is that I've kind of grafted the -lm flags into the src/CMakeLists.txt file, maybe there is a better way? Also there may be some question stylistic choices in BindMathLibrary().

Any input is appreciated! :)