Update to compile with rust master at 4bc7672

Lots of memory corruption when running tests though.
Fix buffer allocation in `to_str_radix`

This is what was causing the memory corruption.
Import num::ToStrRadix and from_str::FromStr

Seems like they were removed from the prelude.
handle vec -> slice renaming
Add a simple build system
Add a divides method on Mpz
Refactor Ord and Eq a bit
Add modulus function

The number-theoretic a mod b. Result is always positive. Examples:

 3 mod 7 = 3
 10 mod 7 = 3
 -4 mod 7 = 3
Add div_floor and mod_floor
Fix divide-by-zero logic

It was checking the numerator instead of the denominator.