Improved ButtonCore's handling and preservation of HTML labels
Added tests to ensure nested labels behave correctly
Added labelHTML ATTR to Y.Button and Y.ButtonCore, and restricted the label ATTR to non-HTML.
Updated HTML label tests to reference the labelHTML ATTR
Added escape to ButtonCore dependencies
Documentation updates.  get('label') now returns the 'text' attribute of its host node.
Added test
Fixed failing Selleck tests
Fixed failing tests in IE
Merge branch 'dev-3.x' into button-labels
* Enhancements to ButtonCore allowed for simplification of Y.Button by removing most of the now unneccesary prototype properties.
* `Y.Button.ATTRS` now points to `Y.ButtonCore.ATTRS`
* Improved code documentation
Updated HISTORY
Locally scoped some objects hanging off Y
Moved ButtonCore private utility methods to static privates.
Added additional tests
Improved labelHTML ATTR's security warning.
Updated Y.ButtonCore's 'label' ATTR getter to HTML escape returned value
Improved labelHTML ATTR's security warning.
Using nodeName instead of tagName.  See: http://www.quirksmode.org/dom/w3c_core.html#t23
Removed unneccesary escaping in Y.ButtonCore's  ATTR getter.