Merge pull request #5 from Vagabond/adt-req_headers_to_list

Add wrq:req_headers_to_list to expose headers in a normalized form
use new Yaws merge_header capability

To properly handle multi-value headers, especially Set-Cookie headers, use
the new Yaws yaws_api:merge_header function.
Fix for cowboy header fetching and socket recv
Fix set-cookie header merging