Add very rudimentary templates.

Templates are a very site-specific thing and these are essentially throw-aways. They demonstrate the basic type-based folder structure, overridden type-specific versions and the `default` fallback template (which also throws a useful JavaScript console error).
New feature - handy templatetag for resizing large <iframe>s. Including tests and example usage.
Update tests - Twitter has been removed from Embedly.

See:
http://support.embed.ly/entries/23883243-Twitter-Embeds-Removed
http://blog.embed.ly/post/48863115964/separating-from-the-flock
New Backend - added a simple Twitter Backend that provides a boilerplate script embed tag.

Now that Embedly does not support Twitter, we need a solution for this common service. This is a very limited backend. Anything more requires authenticating and integrating with the Twitter API.