Add #14
Add #16. Quite ugly, but it works; as far as I can tell, bigint still has several drawbacks. (See code.)
Add #20, and add fac(n: uint) -> BigUint to exscape.rs
Add #21; add divisor_sum() to exscape.rs
Fix #10
Rename isprime to is_prime and fix bug for n < 2
Add rustdoc to exscape.rs
Fix bugs found while creating and testing rustdoc.

1) primes_up_to now returns primes up to *and including* n, as intended.
2) primes_up_to now also only returns primes
3) fac(0) == fac(1) == 1, as it should be
Add doc to gitignore
Checks -> Tests for doc consistency
Add build-doc