Implement empty options for datetime selects.
Change how options are used in Datetime widget and expand tests.

Stick with Florian's original idea of having separate option sets for
each input. This allows additional HTML attributes to be passed through
to the various input methods, and removes the need to prefix each
option. Doing this will make it easier to build an abstraction in
FormHelper that provides a more backwards compatible interface
implementing dateFormat and timeFormat.
Add more tests for ordering the year select.
Implement year range extension.

When the selected value is outside of the year bounds, the year range
should be extended to include the selected value.