Generate sitemap.xml and add robots.txt.
Improve fix from 098369b33f694becf53fcdd9535c631e3f18a062.

Remove extra newlines which aren't needed.
Simplify license.html.

Remove the custom `replace` filter and use `markdownify` instead.
Revert 26bb900113ad6385c743b7a6d61c5595364d075b.

Jekyll already takes care of those.
Simplify quotes and apostrophes.
index.html: specify permalink.

This way we don't get `http://choosealicense.com/index.html` in sitemap.
sitemap.xml: exclude pages with layout: nil.
Merge pull request #181 from XhmikosR/markdownify

Markdownify
Merge pull request #182 from XhmikosR/sitemap

Generate sitemap.xml and add robots.txt.
Move css, img and js files to /assets.
Move source files to /source.