csv-export plugin: improved field lookups

* I changed csv-export to obey the displayName property from columnDefs, if present
* I changed csv-export to use UtilityService.evalProperty for field lookups (before calling the columnOverride function, if present). This is necessary to be consistent with ng-grid. For example, ng-grid allows field keys like "someObject.property" or even "someObject.array[0]"; if csv-export doesn't do the same thing, then csv-export can only work when all field keys are simple property names.