[patch][ccla] Allow Tree to use custom TreeNode implementations

[rcoup]

Currently if you want to make your nodes more complicated than adjusting the CSS or text label it requires monkey-patching dijit._TreeNode.

The attached patch adds a nodeType parameter to dijit.Tree which takes a constructor function (or string that can be resolved), which is used for constructing nodes in that tree. A simple test is there as well. The constructor function needs to be compatible with dijit._TreeNode obviously. But it allows easy behavior changes, using different templates, storing more state, etc.

Submitting for review, since I'm not a Tree expert. I'm also not sure about the nodeType name, feel free to offer improvements.

[rcoup]

minor formatting/style improvements

[bill]

Heh, I'm never sure what to name things myself either. See #5719 which is a dup of this one.