[#17] Improve automatic PDF generation

This is incomplete, but it’s a big step forward.

\*\* You can see the output (for all languages) here: http://codeclub.treadsoft.ly \*\*

Both wkhtmltopdf and phantom.js should work. You can test pdf generation with commands of the following form:

```
python build.py --pdf wkhtmltopdf uk lessons/scratch output/codeclubuk
```

(For usage instructions, type: `python build.py -h`)