Added ng-switch in tree template,to handle large dataset.If large datase...

...t is provided to treeview template it will become too heavy and user interaction becomes much slower,to reduce that I have added ng-switch in template.Now child nodes won't be created until user clicks on parent nodes.when user clicks on parent node. node.collapsed property becomes true and it will generate the child nodes.Such a way this tree directive can handle large datasets(4000+)

Signed-off-by: Jay Patel <jaypa@cybage.com>