Rewrote isPrime function for some problems.
ID 41: Impremented faster permutation.

Implemented function permutations' which lists permutations
guaranteeing order without sort.
On an interpreter, this runs slower than the previous version
since permutations' will not be optimized, but runs faster
if optimized by compiler.
Merge branch 'master' of github.com:yoh2/projecteuler-ans