Antidivisors

Implemented functions to compute antidivisors and the number of
antidivisors.  Antidivisors are the numbers that do not divide a number
by the largest possible margin.

References
==========

- [1] http://oeis.org/A066272/a066272a.html

added as_int checks + changed reference text
added as_int to definition of divisors() as well.
fixed formatting
changed doctest example to sorted list
removed imports from __init__
Merge branch 'antidivisors' of https://github.com/postvakje/sympy into antidivisors