Adding four arithmetic functions

[reinermartin]

I have added 4 functions: moebius, liouville, divisorcount, and divisorsum. They are all multiplicative in the sense that f(n m) = f(n) f(m) if n and m are relative prime, and they can easily be computed from the prime factorization (see https://en.wikipedia.org/wiki/Arithmetic_function#Multiplicative_functions). Documentation and tests are included.

This is functionality for which a system like Mathematica has been useful in the past, but it would be good to have it in Julia. I plan to add more such functions and more detailed documentation later.

[reinermartin]

I don't know how (technically) I can resolve this trivial conflict in api.md (the version where the 4 new functions are present is the valid one). Am I even allowed to do it? Help!

[fkastner]

I'd also like to have this (esp. the Möbius μ). Any chance to revive this PR?