fixed invalid case for Array.first
refactored Array.first to use slice to get first n elements
added test for Array last if invalid number is passed,refactoring
added test and refactored Array.last if valid number is passed
added tests for Array.filter
added test for Array.reverse
refactored Array.reverse
clean up
Merge pull request #3 from christian-fei/master

New Tests and improvments to Array.reverse and Array.first