Make the main command print all tables by default

This should deprecate 'launchpadstats-all'. All the tables are printed if no
'--table-name' is specified.

A bit of behaviour changed as well:
- in CSV output, the description of the table is shown in a comment before the
  table
- in HTML output, the <html> header and footer are shown even for a single
  table
- show <h2>table description</h2> even for a single table
Removed launchpadstats-all

The same behavior can be now achieved with 'launchpadstats -o html' without
specifying a table name.