OEmbed: Change meta and headers to camelCase, implement cache and improve error catching

packages/rocketchat-oembed/client/baseWidget.coffee

packages/rocketchat-oembed/client/oembedUrlWidget.coffee

packages/rocketchat-oembed/client/oembedYoutubeWidget.coffee

packages/rocketchat-oembed/client/oembedYoutubeWidget.html

packages/rocketchat-oembed/package.js

packages/rocketchat-oembed/server/server.coffee