Refactor website (#146)

* Migrate to using "remote themes" instead of forked content
* Update to most recent mmistakes
* Add docker-based testing
* Create Makefile for test targets
* Add travis-ci integration
* Update badges to use shields.io
* Replace splash page image (old one's attribution disappeared from
  unsplash.com)

test/travis-ci

@@ -0,0 +1,22 @@
+#!/bin/bash
+set -e
+
+msg() {
+  echo -e "\e[1;32m$*\e[0m"
+}
+
+gem install html-proofer
+msg Success: Installed html-proofer
+
+jekyll build -t --config _config.yml,_dev.yml
+msg Success: Built jekyll site
+
+jekyll serve --skip-initial-build --detach
+msg Success: Serving jekyll site locally
+
+/usr/gem/bin/htmlproofer \
+  --assume-extension \
+  --log-level=debug \
+  --http-status-ignore 429 \
+  _site
+msg Success: Validated site with html-proofer