Change data.$headers to a collection of header cells.
Change data.$headers to a collection of header cells.

This avoids the odd usage where a row was stored but only the contained cells were used.

Removed the headerRow class and added the sortHeader class, which is added to column headers that have sorting enabled.

edelooff/timbles.js,jennschiffer/timbles.js,edelooff/timbles.js,jennschiffer/timbles.js