feat(jekyll): Major update for Jekyll

Changed the theme to Jekyll from the default theme to Lanyon based on
Poole from mdo. Also added a couple other files needed for a basic
website: robots.txt, sitemap, atom feed etc. Updated the gulpfile and
config.yml files accordingly.

Also some minor changes here and there for some other files, including
asking the user for a URL to their site used when making the .xml files
and such. This bumps the version to 0.4.0.
docs(changelog): Updated the changelog