Add Netlify configuration.

- The netlify.toml file explains how to build this site for production
  and deploy previews.
- The Makefile has some commands to make building the site slightly more
  user friendly, try typing `make help`.
- The Gemfile includes all the dependencies exported from GitHub Pages.
  This site might not need all of the but I don't have the knowledge to
  exclude the unused ones.

Signed-off-by: David Calavera <david.calavera@gmail.com>

Gemfile

@@ -0,0 +1,20 @@
+source "https://rubygems.org"
+
+gem "jekyll", "3.2.1"
+gem "jekyll-sass-converter", "1.3.0"
+gem "minima", "1.1.0"
+gem "kramdown", "1.11.1"
+gem "liquid", "3.0.6"
+gem "rouge", "1.11.1"
+gem "jemoji", "0.7.0"
+gem "jekyll-mentions", "1.2.0"
+gem "jekyll-redirect-from", "0.11.0"
+gem "jekyll-sitemap", "0.10.0"
+gem "jekyll-feed", "0.5.1"
+gem "jekyll-gist", "1.4.0"
+gem "jekyll-paginate", "1.1.0"
+gem "jekyll-coffeescript", "1.0.1"
+gem "jekyll-seo-tag", "2.0.0"
+gem "jekyll-github-metadata", "2.0.2"
+gem "listen", "3.0.6"
+gem "activesupport", "4.2.7"