feat(MdDatepicker): add immediately option

[VdustR]

Main feature

Click to select date without confirm and close the dialog immediately.

Other changes

• If disable MdImmediately, date would not be selected before click confirm button. Before this change, confirm button is useless.
• Selected date in datepicker will reset when cancel button clicked(on the dialog closed).
• setContentStyles is always called when transition start. I think this way is better than setTimeout.
• fix clear button did not clear selectedDate bug.

fix #1606