Added YouTube adapter to always fetch SSL'ed embed code

Currently, the embed codes for YouTube videos will always try to load the videos via an unencrypted connection. Therefore it is not possible to use the embed codes generated by this library in a SSL-encrypted application.

YouTube's oEmbed endpoint explicitly requires specifying the `&scheme=https` parameter. Without this parameter the endpoint will always return URLs having the `http` scheme (the endpoint URLs in YouTube's `link` tags do not contain this parameter).

See this WordPress bug: https://core.trac.wordpress.org/ticket/18719#comment:11

This pull request introduces a new adapter that will force YouTube to always return embed code that loads the video using an SSL-encrypted connection.