Sharing of Element styling across Survey and Element classes is confusing

[mdahardy]

In version 0.2.1, the default styling for the elements is done in the Survey class. However, the element styles merging - along with the styleKeys and selectorMap - is done in the Element class. This seems confusing and prone to bugs - we should update Survey and Element so that the default Element styles and Element style functionality is done in the same class.