Added support for dynamic binding and multiple DownPickers in single controller

[EccelorRavi]

Added support for dynamic data binding using datasource and delegate

Use DownPickerDataSource to provide dynamic data to DownPicker Use DownPickerDelegate to receive various events,

1. didSelectItem - provides event when any item is selected in picker
2. didPickedItem - provides event when done button is clicked and final selection is made
3. didCancelSelection - provides event when selection is cancelled

All the delegate method provides DownPicker instance to support multiple DownPicker events in single controller

[EccelorRavi]

Making a few tweaks