Date Picker defects while used in GC

[ericsewalson]

2 defects:

1. Passed in selected value cannot be overridden by selecting a new date.
2. Need to add an onSelect prop instead of onchange to account for the user selecting today or the same date. This is not important for future dates but is relevant when selecting today as the date since the value defaults to today (and doesn't register an onChange event if the date is not changed from default)