Refactor out helpers package

[rexim]

Derived from #204 (comment)

[ForNeVeR]

For the record (GitHub didn't properly show the linked comment), the comment text is:

"Helpers" and "Utilities" are instances of coincidental cohesion which is the worst possible cohesion according to Larry Constantine https://en.wikipedia.org/wiki/Cohesion_(computer_science)#Types_of_cohesion

I will create an issue to eliminate this package, but for I now I strongly recommend to create cors package and move everything related to CORS there.