Rename normalize.scss as a partial

We don’t want it being output as a separate file in our build directory
as it should all compile into a single all.css file.
Config: activate minify_css
Gems: add middleman-livereload gem

I know now what was missing that caused this not to work earlier. We
need to activate it in the config.rb.
Config: activate livereload plugin
Config: activate automatic_image_sizes
Update pages to use image_tag helper with no sizes

New config setting will automatically detect images sizes and add width
and height attributes.