Layout/CSS changes needed to link posts/projects

Resolves #108.  Uses technique to nicely crop existing project images,
but would probably be better in the future to use smaller project
images (#164)
Updated guidance on adding blog post (see #108)

Optionally add “project:” to front matter
Adds "projects" metadata to some posts

Links posts to projects as in #108
Merge pull request #234 from danfowler/link-posts-to-projects

Link posts to projects