Remove .bundle from version control.

From

    http://bundler.io/v1.5/bundler_sharing.html

>Do not check in the .bundle directory, or any of
>the files inside it. Those files are specific to
>each particular machine, and are used to persist
>installation options between runs of the bundle
>install command.
Fix typo.
Add Nokogiri and other gems.

Ok, nanoc check needs nokogiri, :relativize_paths
needs nokogiri. I give up, I install the behemoth.
I won't keep gems local to my projects any more.
Add (commented) :relative_paths filters.

To enable relative paths, uncomments those lines.

This does not look very dry, think about defining
a helper function.
Put highlight.js in HTML header.

The link tag in the highlightjs.erb snippet must
be put the header, otherwise it does not validate.
Fix nested paragraph.

The summary() function alread returns a properly
tagged paragraph.
Fix minor inconsistencies and unescaped characters.
Ignore .bundle directory.