Update application layout to use bootstrap 3.

Update the application layout to use new bootstrap 3 components and
styles.  Add a main link to the top nav if signed in.  This is to make
the top nav consistent throughout the application if a user is signed
in.
Update blog post partial for bootstrap 3.

Update the blog post partial _blog_post to work with the new styles in
bootstrap 3.  Modify the application layout to yield and display the
blog post content.