Move HTTP client to its own package

http/client/client.go

http/client/response.go

http/client/response_test.go

http/doc.go

integration/instapaper/instapaper.go

integration/nunuxkeeper/nunuxkeeper.go

integration/pinboard/pinboard.go

integration/wallabag/wallabag.go

reader/feed/handler.go

reader/icon/finder.go

reader/scraper/scraper.go

reader/subscription/finder.go

ui/proxy.go