New: `data` and `render` options for columns support function notation

- As part of completing the planning development for reading data, I've
  added support for calling functions from the string defined in `data`
  and `render` column options. So you can now do something like:
  `render: 'name()'` rather than needing to use an anon function and
  calling name() in that. This is useful for cases where you want to
  give DataTables an array of Javascript instances, rather than objects
  or arrays (see example below). It also fully supports the continuation
  of the dotted notation DataTables supports, so you could use
  `name().first` if `name()` returns an object. Again to make it easier
  than needed to define a function.

- Documentation for `data` and `render` updated to reflect this
  abilities

- Unit tests for this still to come

- There is one backwards incompatiblity that should be noted - although
  I think this is a real edge case and I just can't see it being an
  issue. If before, you had `data:null` without `render` or
  `defaultContent` specified, DataTables would have output an empty
  cell. Now it will output the original data source object. Can't see
  this being an issue since, why would you have a column empty cells? If
  this is an issue, then you simply need to add `defaultContent:''` now.

- Example use case, using Javascript instances:

$(document).ready(function() {
  var z = function (i) {
    this.a = function (set) {
      if ( set ) {
        return this;
      }
      return i+'-0';
    };
    this.b = function (set) {
      if ( set ) {
        return this;
      }
      return i+'-1';
    };
    this.c = function (set) {
      if ( set ) {
        return this;
      }
      return i+'-2';
    };
    this.d = function (set) {
      if ( set ) {
        return this;
      }
      return i+'-3';
    };
    this.e = function (set) {
      if ( set ) {
        return this;
      }
      return {
        q: i+'-4q',
        w: i+'-4w'
      };
    };
  };

  window.dataset = [
    new z(0),
    new z(1),
    new z(2),
    new z(3),
    new z(4),
    new z(5)
  ];

  $('#example').dataTable( {
    columns: [
      { data: null, /*render: 'a()'*/ },
      { data: 'b()' },
      { data: 'c' },
      { data: 'd()' },
      { data: 'e().q' }
    ],
    data: dataset
  } );
} );