New: Give cell position information to `dt-init columns.data` and `dt-init columns.render`

* The two data handling functions for each column are now given a forth
  parameter if you are using them as a function. This new parameter
  gives index position information about the cell in question, as well
  as access to the settings object.
* This additional information allows abstraction functions to be created
  external to DataTables that can be reused for different columns, with
  those abstraction functions now having access to the information about
  the cell they are operating on. For example, you might have a number
  formatting function which can be reused, and it will determine what
  data to read based on the column index given.
* This additional information is required in order to be able to fully
  replace fnRender which was removed in DataTables 1.10.
* This fixes DataTables/DataTables #321
* Documentation updated, including an error fix for columns.data