Adds support for Oneboxes without Layouts

Currently the onebox gem assumes you want to use a Mustache layout for your onebox result. While most of the time this is true, there are certain kinds of embeds where it doesn't make sense. This pull request focuses on the onebox for Vimeo as an example, where you'd rather just embed an `iframe` than render a layout.

This pull request does the following:

- Extracts the Layout capability to a mixin, `Onebox::LayoutSupport`
- Adds a new capability, 'Onebox::IFrameSupport` for rendering an iframe from opengraph results
- It renames `template_name` in the `Engine` to be `onebox_name` since we won't always have templates
- I added the dependencies for `_onebox.rb` classes (`html`, `open_graph`, etc) to the classes themselves.

Let me know how it looks!