support html5 date input

[bboyle]

initial support for type="date" (not datetime, time, month or week—they will be implemented later as needed)

will not set @value to YYYY-mm-dd format. Applications must handle input in varied format as if type="date" was not supported (fallback in browsers is type="text")

will accept any value that can be parsed using new Date( _value_ ) and will polyfill support YYYY-mm-dd formats if not natively supported by the javascript engine.

add .valueAsDate which will be a javascript date object. Will be updated onchange
http://www.whatwg.org/specs/web-apps/current-work/multipage//common-input-element-attributes.html#dom-input-valueasdate

add valueAsNumber as an alias for valueAsDate.getTime()

[bboyle]

Handle browsers that support validation API but do not support date input.
e.g. Firefox 14

Cannot edit .validity ValidityState object to set .typeMismatch to false on bad date input.
Can replace .checkValidity() method with custom function (but should use native where possible).

Use custom validity?

[bboyle]

Does using custom validity belong in the polyfill file or should it be moved to another script?