Disabled checkbox should not be in the tab order

[marcysutton]

To better align with native checkboxes, mdCheckbox components using ng-disabled should not have tabIndex=0. A native checkbox will not be part of the tab order when the disabled attribute is used. The Material Checkbox should have similar behavior.

@ThomasBurleson started PR #1168 for ng-disabled improvements for mdButton, but I wanted to capture the mdCheckbox tabindex issue. mdSwitch does not suffer from the same problem.

See the "mdCheckbox" section in this Codepen: http://codepen.io/marcysutton/pen/yyxREg
Compare it to this native checkbox Codepen: http://codepen.io/marcysutton/pen/GgxboO

[ThomasBurleson]

Related to #2297