Add Cocktail Sort

[migueldalberto]

The cocktail sort (also known as cocktail shaker sort, bidirectional bubble sort, shaker sort, ripple sort, shuffle sort, or shuttle sort) is a variant of the bubble sort algorithm. It extends the bubble sort by operating in both directions (from array's beginning to its end, and from array's end to its beginning).

I based the implementation on the Wikipedia page about Cocktail Sort.

Also added this sort on the sort_tests.go

[raklaptudirm]

Add some inline comments explaining the algorithm as far as possible.

[raklaptudirm]

I would suggest adding start and end variables to make the sort more efficient.