Add a rake task for setup git to allow github pages publishing

It's quite similar to this:
http://nanoc.ws/docs/guides/deploying-nanoc-sites/#with-github-pages
Add crash.log to gitignore
Add git deployer with its configuration
Add Procfile
Add .env to gitignore
Use thin as development web server
Add slim and markdown
Add guard-nanoc
Improve git deployer a bit to avoid commiting when repo is clean
Add strip_blank_lines filter