Add support for iframes when converting to markdown

iframes were being stripped out of the markdown output. This includes them.
I also removed some unused dependencies (upmark) and methods (sanitized_content).
Merge pull request #10 from monfresh/add-support-for-iframes

Add support for iframes when converting to markdown
add export/ to gitignore
Merge pull request #12 from bensheldon/gitignore_export

Add export/ to gitignore
Merge branch 'master' into to_yaml_for_frontmatter
update iframe spec expectations to to_yaml format