added n <= k condition to combinations function

and also changed x to n in combinations
changed x to n in permutations
Changed parseInt to Math.floor

also added vim swap files to .gitignore
added BigNumber support to permutations (with tests)
Added BigNumber support to combinations (and tests)
added n <= k condition to permutations, combinations
Tests for non-integer bignumbers in combinations
Updated combinations/permutations documentation
Merge pull request #129 from daniel-levin/develop

Permutations and Combinations functions now accept BigNumbers