[MERGE] improve pivot table rendering

* don't redraw the full table when folding/expanding rows: simply removing/inserting
the rows from/into the DOM is faster
* add a $ in front of jquery variables
* improve the way indentation in row headers is done (was done by generating useless spans)
* fix a stupid bug: a tbody tag was added around each row of the table

bzr revid: ged@openerp.com-20140509110350-al7phvo3wk5rozya