Overriding Extra Rest params prior to base.OnInit.

In each of these pickers, ItemRestUrlExtraParams was being overridden
after the call to base.OnInit in the control's OnInit method, thereby
negating the fact that the property was being overridden.

 - CategoryPicker
 - Group Picker
 - LocationItemPicker
 - PagePicker
Tweaks to item picker client and server code.

 - Updating client code to accept more parameters and pass to rockTree
   jQuery plugin.
 - Updating server code to render out corrected class names.
Refactoring JS to use RockTree, syntax corrections.
Refactor rockTree to pre-fetch expanded nodes.

When option "expandedIds" is supplied, the plugin will attempt to prefetch
all expanded nodes. Rendering will be delayed until all successive
requests to load data are complete.