New: Instance methods for a data source are now executed in the scope of that instance

* When `dt-init columns.data` / `dt-init columns.render` resolve to a
  function, that function is executed. Previously this execution was in
  the global scope, but in order to be able to use an instance as a data
  source for the row (something I wanted as part of 1.10) we need to
  execute the function with the instance's scope. A simple `call()` does
  this.
* I could add a check to see if the `rowData` is a plain object or not,
  but that would add significant overhead to the processing, and this
  particular part of the code needs to be very fast. Additionally, I
  think it is unlikely that any one was dependant on this being executed
  in global scope, so the change is made.
* This fixes DataTables/DataTables #368.