Make TextField value handling match React.DOM.input value handling

[robertknight]

Make the way TextField handles setting its value match the way React.DOM.input and other form controls work.

If props.value is supplied, then the component is a 'controlled' component and its value always matches props.value, otherwise it is an uncontrolled component whose initial value is props.defaultValue.

[SanderSpies]

Thanks for this improvement.