Breadcrumbs for is_active_page

I had the following use case:

Some pages have a hierarchy structure. Not all of such pages will be in top navigation bar. I wanted the appropriate parent page link to appear "active" if any descendent is the current page.

Eg:
Blog (index.html) -> Some Blog Post (some-blog-post.md)
In above case, I would like "Blog" nav link to appear active when viewing "Some Blog Post".

I have introduced "breadcrumbs" YAML metadata to enable above behavior. 
Eg:
some-blog-post.md
-----------
breadcrumbs: ['index.html']
-----------

Please merge if appropriate for everyone.

I like to TDD. Noticed that a LOT of specs were deleted on 5 Jan 2013. Not sure of your unit-test-related plans going forward, so I have committed my specs in a separate commit just in case you don't want the specs. Please let me know for future pull requests.

Thanks.