Fix phantom.js build command
Only generate a pdf if we’re not copying one.

There’s no point copying *and* generating a pdf. Let’s do one or the
other.
Use npm-installed version of phantomjs.

Versions newer than 1.9.2 appear to have font problems, so we set an
upper bound on the version for now.
Add pandoc pagebreak filter
Add a custom PDF footer with phantomjs
Add some extra html markup via javascript.

In order to style up some elements for pdf generation, it’s necessary
to add some spans via javascript. Some of this can be fixed by
modifying lesson markdown / manifests, but it’s perhaps simplest to
do it this way.
Add a legend template; move it into position in js
Lots of new PDF styles!
Move js run by phantomjs to assets/js; rename
Undo 3800f2b5; add pagebreaks with an extra class.

The pagebreak filter was messy and a bad idea! I should have done it
this way in the first place.
Get wkhtmltopdf working as well
Add a command line switch to enable pdf generation
Remove all pdf-specific stuff from generated HTML

 - Split PDF-specific styles out into separate files
 - Only load PDF styles when we’re rendering a PDF
 - Similarly, only load PDF-specific js when we’re rendering a PDF
 - Reinstate original print css
Generalise rasterize.js

 - Add an option parser
 - Add --footer, --style and --script args
 - Import footer content from a template file
pdf.js shouldn’t make assumptions about html
css tweaks (based on inspecting output)