Fix - docs: Error in examples for `xhr` event
New: Cached footer cells can now have colspan properties, allowing
column index referencing of the cells.

- Previously a colspan in a footer cell would cause index misalignment -
  for example if you had a table of 6 columns with two in the footer
  (one colspan=5) the fotoer cells would be assigned to the first two
  columns in the table, breaking column index order. Now a cell can be
  used over multiple columns. This allows column().footer() to always
  reflect the cell that belongs to that column (possibly sharing it with
  other columns).

- Note that the footer will only use cells from the first row in the
  footer. If there is more than one row, use table().foot() to access
  the TFoot element and then manipulate it as needed.
Merge branch '1_10_wip' of github.com:DataTables/DataTablesSrc into 1_10_wip