* Adds new page /en/blog
* Adds new feed for blog posts to /en/rss/blog.xml
* Adds "Blog" link to site footer
* Adds three back-dated blog posts to give the blog some initial
content.
Uses Ruby html-proofer to check the links. This commit also fixes the
various problems it found, as well as dealing with some of its
non-problem complaints (it doesn't like anchor (a) tags without either
an href, name, or id).
Running HTML proofer takes about 12 minutes on my system (with up to two
threads), during which it prints no text. Travis CI times out after 10
minutes of nothing being written to stdout, so this commit also adds a
background process the Makefile to print a line every minute while make
runs.
