Allow static files in collections

Allow Documents to be static files so static files can exist in collections
Add blank frontmatter to feature test
Cache information about document YAML header existence
Add tests for static files in collections
Move instances of `Time.parse` into a Utils method
Merge pull request #2615 from alfredxing/collections-static-files
Update history to reflect merge of #2615 [ci skip]
Show input in error message

Show date input in error message to make it easier for user
to find infringing date/file
Add tests for `Utils.parse_date` method
Link to jekyll-slim

Link to https://github.com/slim-template/jekyll-slim
Reworking site template to utilize Sass
Update welcome post to clarify front matter usage
Replacing CSS comments with Sass comments
Adding and using media-query mixin
Removing unused column class
Adds Smashing Magazine tutorial to resources page
blockquote needs spaces before it
Removing h1 from site-title
Adjusting heading hierarchy to reflect site-title change
Merge pull request #2689 from minad/patch-1