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
use to_yaml instead of handbuilt serializer
Merge pull request #12 from bensheldon/gitignore_export

Add export/ to gitignore
Add support for comments

Some Wordpress posts use <!--more--> to specify where the excerpt should stop. This commit preserves those comments.
Merge branch 'master' into to_yaml_for_frontmatter
update iframe spec expectations to to_yaml format
filter file attachments from exported posts
Merge pull request #13 from bensheldon/ignore_attachments

Filter file attachments from exported posts
Merge pull request #15 from monfresh/add-support-for-comments

Add support for comments
Merge branch 'master' into to_yaml_for_frontmatter

Conflicts:
	spec/lib/wp2middleman/migrator_spec.rb
Merge pull request #14 from bensheldon/to_yaml_for_frontmatter

Use to_yaml instead of handbuilt serializer