Add blog functionality

Add configuration for blog functionality, and style changes for blog entries, tag lists, and post lists.

Note that on /blog.html, <%= link_to article.title, article.path %> functionality depends on the :relative=>false option. If this flag is omitted, going to portlandcodeschool.com/blog (no trailing slash) will build article links incorrectly and all article links will 404.
Add link to blog to navbar
Fix gap in footer background color at smallest size.
Switch the position of current classes and CWK on the index page
Change Data Viz icon from coming soon to now enrolling
Add Mobile Development with iOS to the site. Rearrange classes page to reflect new content.
Add placeholder content to blog page until I have content