WIP user guide for promises and example stubs
Add Tree.Sortable extension.

Mix this into any Tree-based class to provide customizable sorting for
nodes.
Tree.Sortable docs.
Move Tree.Sortable history entry to the right version.
Fix lint warnings.
First draft of Promise user guide complete
Good enough to ship. Needs examples, diagrams, and editing
Remove unused vars in Tree tests.
Tree: Add traverseNode() and findNode() methods.

…along with matching traverse() and find() wrapper methods on Tree.Node.

traverseNode() passes the specified node and each of its descendants to a
callback function in depth-first order. Traversal can be stopped by returning
`Tree.STOP_TRAVERSAL` from the callback.

findNode() passes the specified node and each of its descendants to a
callback function and returns the first node for which the callback returns a
truthy value.
Fix `Y.clone()` to quit early when trying to clone DOM nodes

This moves `Y.clone()`'s early return for DOM nodes above the call to
`Y.Object()`, and fixes the issue where DOM nodes cannot be cloned in
Firefox or IE.
Format Handlebars README
Update Handlebars' Gruntfile to work with Handlebars @ v1.0.10
Removes "BEGIN/END(BROWSER)" comments from imported Handlebars source
Tweak Handlebars Gruntfile more to make it smaller and more efficient
Add notes about why we don't `Y.clone()` DOM nodes
Wrangle Y.Clone's `var`s
Remove the default build directory from loader combo url
Added HISTORY entry
Merge branch 'dev-3.x' into tree-sortable

Conflicts:
	src/tree/HISTORY.md
	src/tree/tests/unit/tree.html
Build tree.