Allow to use datepicker specific setDate arguments

Change is to support the '-1d', '+1m', and such datepicker.setDate arguments with the overridden setDate function.
Merge pull request #713 from aokros/dev

Allow to use datepicker specific setDate arguments
Add support for ('setDate', null)

When ('setDate', null) is called it deletes only date part of alternate field but time still remains.
This patch should fix the problem and properly delete whole alternate field in case that null is set as new date.
Fixed wrong commit
Merge pull request #714 from jhron/patch-1

Add support for ('setDate', null)
Fixes #706 altTimeFormat, altSeparator, altTimeSuffix doesnt allow empty value
Merge commit '9a28b917055f988bcdf3c7d030087dc7eab1a05b'

Conflicts:
	dist/jquery-ui-timepicker-addon.js
	dist/jquery-ui-timepicker-addon.min.js
	src/jquery-ui-timepicker-addon.js
Cherry-picked "Accept lang property."