This is a large commit. The cowboy_req interface has largely
changed, and will change a little more. It's possible that
some examples or tests have not been converted to the new
interface yet. The documentation has not yet been updated.
All of this will be fixed in smaller subsequent commits.
Gotta start somewhere...
Along with more cowboy_req tests.
This commit also removes cowboy_req:url/1 and cowboy_req:host_url/1
in favor of the much more powerful new set of functions.
