Move `update` functionality to `setDate` and remove `update`

As evidenced by a rash of related bugs (#399, #466, #544, #554, #556), people expect setDate to behave as update does (and, not coincidentally, how jQuery UI datepicker’s setDate does) – despite the documentation clearly stating that setDate “sets the internal date”, nothing more.

[omidkrad]

I see no use case where you want to setDate without getting the UI updated.

[razvanphp]

Version 1.4.0

• update function does not update the UI if you pass a date object (works only with a string)
• update function never triggers changeDate
• .datepicker('setDate', new Date()) does not return the object for chaining another datepicker method
• setDate still does not highlight the current date for the inline version