Added a little description for the user
Changed some names, added a record-id and a few more

The onclick configuration option is now onCellClick to tell it appart
from the raw DOM event.
A recordId configuration attribute has been added to store the
primary id of each record in each row.
ClassNames for columns are now applied to header rows as well as data
cells.
Added one more check to the detection of clicks on the sort icon.
Change some internal (non-public) names which might have been confusing.