Improve pre-checker reporting format (smurf file)

This is about to add some modifications to the smurf.xml file (and its xlst conversion to html) bringing us:

• counters (by check and total) of errors and warnings. This will help later (see MDLSITE-3423) when reporting information to the tracker in a more informative way.
• some tweaks to current html transformation, improving formatting, better separating error and warnings, cleverer line # handling, avoid file repetitions...

And that is it... let's go.