Updated default layout to include header image

When an header image is defined inside the YAML front-matter
of a page (key: "header") it is included in the page right after
the title.
gitignore: removed asset/css/app.css, now generated below _site
Makefile: reorganized targets
Merge branch 'master' of github.com:lvillani/quickstart-jekyll-zurb
Merge branch 'master' of /home/lorenzo/Development/HTML5/quickstart-jekyll-zurb
Makefile: updated deploy target
Merge branch 'master' of /home/lorenzo/Development/HTML5/quickstart-jekyll-zurb
quickstart: update generated assets for Github Pages
Makefile: clean before deploy and ignore errors
Merge branch 'master' of /home/lorenzo/Development/HTML5/quickstart-jekyll-zurb