Encapsulate the oEmbed -> oohembed fallback into oEmbedHelper class. Also added a...

Encapsulate the oEmbed -> oohembed fallback into oEmbedHelper class. Also added a chance to whitelist sites that don't show discovery info but do have oEmbed API endpoints, and to provide alternate APIs for some common services.

Newly supported:
- TwitPic: added a local function using TwitPic's API, since the oohembed implementation for TwitPic produced invalid output which Services_oEmbed rejects. (bug filed upstream)

Tweaked...
- Flickr: works, now using whitelist to use their endpoint directly instead of going through oohembed
- Youtube: worked around a bug in Services_oEmbed which broke the direct use of API discovery info, so we don't have to use oohembed.

Not currently working...
- YFrog: whitelisting their endpoint directly as the oohembed output is broken, but this doesn't appear to work currently as I think things are confused by YFrog's servers giving a '204 No Content' response on our HEAD checks on the original link.