Work on improving pickable entities - Lights Optionally pickable, Spheres & Text pick properly, other bug fixes

Some foundation work to improving picking of entities.

* introduced virtual method for individual entities to do detailed ray picking against their contents
* make "lights" choose to be pickable based on an EntityTree setting and Entities scripting interface that says whether or not they should be pickable
* added support to editModels and newEditEntities to toggle light pickability based on a menu item - lights NOT-pickable by default when those tools are running.
* SphereEntityItem now correctly implements ray picking against its sphere not its bounding box.
* fix a bug in ray picking where we weren't always honoring the "closest distance" entity.
* TextEntityItem now correctly implements ray picking against its plane not against its bounding box.
 
Additional things in this PR:
* cleanup of redundant _displayViewFrustum... this was recently added and wasn't actually adding any value, just complicating the code. moved back to using _viewFrustum
* added a hotkey to "reset" the offset view frustum... TODO: clean this up more, maybe migrate it to a developer script.
* remove dead code in MyAvatar for "mouseRay" and "mouseClicked" - these were not being used and really didn't belong in the avatar class anyway.


With this PR, the following jumble of entities is all correctly pickable... previously you wouldn't be able to untangle these because they were all inside each others bounding boxes.

![screen shot 2014-12-04 at 1 36 03 pm](https://cloud.githubusercontent.com/assets/2093195/5306608/9bb02f86-7bba-11e4-95be-838047bb12a6.png)