* Added directory for custom OEmbed providers, that implemend
OEmbed conversion providers for services that don't support oembed
* Implemented a post.ly custom provider, fetching info from the
Posterous API and converting it into photo or video OEmbeds as an
example for a hadwritten custom provider.
* Added "generic providers" embed.ly and oohembed.com.
* Removed the depending definitions in providers.xml
* Added a "generic provider" configuration with embed.ly API key.