When using aggregates, it should be possible to define your own aggregate function, or even set it globally for kendo, so it could be used in other places. i.e. 

currently you can only specify the function name like 'sum', 'avg', etc. with this idea you could give the whole function like function(arguments...) {}

Also from what I saw it would be nice to have the `functions` object from kendo.data.js accessible, so you could add your custom functions, like:
`kendo.data.aggregate_functions['median'] = function(arguments...) {}`