Get tabIndex, disabled, and onchange working for Slider, and add several automated tests, plus some whitespace cleanup.
Fixes for 2.0 event handling.

1. There shouldn't be any widget methods like onClick() anymore, unless the widget is using it
   internally, but then it should be called _onClick() with an underscore.
2. Widgets already inherit methods like onclick and onchange from HTMLElement,
   but they should define functions like onfoo() for non-standard event names.
3. All listener functions (even onchange) get called with single argument: the Event object.
Fixes and test for parsing attributes during declarative instantiation:

1. Allow globals (ex: store="myStore") for Object attributes.
2. Fix functions attributes like funcProp="console.log(...)"; they were being ignored.
3. Disallow globals for functions, ex: onclick="myFunc", since that's inconsistent
   with how native DOMNodes work, where you need to do onclick="myFunc();"
reenable removeAttribute() call, but fix bug on IE
Additional declarative tests to test the removeAttribute code in Widget::mapAttributes
Get test_Slider.html working with IE10.  Workaround broken IE10 pageYOffset.
StoreMap fixes
fix some doc/comments/code following @wkeese comments
including removing the mappedKeys definition feature
make sure stateful properties are enumerable
remove test that is now useless, remove clone
some jshint & test cleanups
move back Invalidating to not necessarily be a Widget
inherit from Stateful
update CONTRIBUTING.md to be more inline with current project
Bug fix: widget is now correctly initialized in all cases (including
when using default values for all attributes). Tabindex is now correctly
set in all cases.
Automated tests updated to check all the aria attributes and tab index.
Bug fix: now properly supporting tabIndex setting.
remove useless test on tabIndex value.
as we stopped (momentarily?) using setTimeout, stop using remove() accordingly
tabIndex management.
fix README typo and list exact commands to execute