Implement divisor_sigma function

Divisor function [1]_ sigma_k(n) for positive integer n:

divisor_sigma(n,0) is equal to divisor_count(n)
divisor_sigma(n,1) is equal to sum(divisors(n))
divisor_sigma(n,k) is equal to sum([x**k for x in divisors(n)])

Default for k is 1.

References
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.. [1] http://en.wikipedia.org/wiki/Divisor_function
Merge pull request #8185 from skirpichev/sigma-rebase

Implement divisor_sigma function