divisibility tests

[Jason Moxham]

The divisibilitys tests
mpz_divisible_p and mpz_divisible_ui_p give DIVIDE_BY_ZERO errors
when the denominator is zero.

The usual definition of divisibilty is

a divides b iff there exists a c such that ca=b

therefore

mpz_divisible_p(x,z) should return 0

and 
mpz_divisible_p(z,z) should return 1

instead of a DIVIDE_BY_ZERO error

where x is a non-zero mpz varible and z is zero mpz varible

same for mpz_divisible_ui_p and perhaps mpz_congruent's

mpz_divisible is testing that one number is divisible by another , not doing 
any division , so it should not have a DIVIDE_BY_ZERO error.