Add Mercurial lesson as dependence of Makefile
Replace div blocks with block attributes

More information about block attributes at
http://kramdown.gettalong.org/quickref.html#block-attributes.
Improve explanation of Mercurial ignore file
Add "hot to avoid "*.orig" globally
Avoid talking about Git at Mercurial lesson
Fix command of Mercurial lesson
Merge pull request #4 from r-gaia-cs/hg-small-contribution

Small contribution to Mercurial lesson