advanced rendering

This patch tries to introduce "advanced rendering" into DataTables.
"Advanced" is meant to express that the (display) render function is not limited to just returning the new content of a table cell, but rather change (render) the table cell directly.
This includes classes, css style and - of course - the content of the cell.

This is realized by using the call() function to execute the render function in the context of the table cell, e.g. "this" refers to the rendered cell.

Example: