Move Yt.configuration.api_key out of request.rb

First step in making Request agnostic of the rest of Yt
Move Yt.configuration.developing? out of Request

Last step to remove Yt.config from Request, in order
to make Request agnostic of the rest of Yt.
Move YouTube-specific errors out of Request

One more step towards making Request a YouTube-agnostic class.
This commit partially reverts 557058bf which already had the same
behavior outside of Request.
Make Request#request_error_message private

One more step toward extracting Request from Yt, leaving only a
minimal public interface: #run and #as_curl
Extract 'www.googleapis.com' from Request

One more step in making Request a YouTube-agnostic class.
Finished cleaning up Request class

Finally all the methods of Request are documented and agnostic of
YouTube (except :api_key and :camelize_params).

This commit also renames the public method `refresh` on Account to
`refreshed_access_token?`, but this method was never really exposed
as part of the public API, so no major version bump is required.
Move Yt::Models::Request to Yt::Request

A request is not really a model (like Channel, Video), so deserves
its own namespace.
Merge branch 'extract-yt-from-request'