Fix event unbind in multiple datepicker instances.

This commit creates a private copy of the datepicker proxy for
unbinding.  Now, .hide() will only remove this instance's event
handlers.
Switch document back to mousedown.

Switch the global hide listener back to mousedown.  This effect is nicer
when switching from one datepicker to another, as it won't temporarily
display both.  It also fixes problems where click starts another
datepicker input but ends somewhere outsideClick won't trigger.
Handle hiding when tabbing between controls.

Add a focusin event handler to close the datepicker when focus changes
to another input, such as when tabbing.  Previously, inputs would be
left open.
Merge pull request #324 from cesutherland/fix/#323-closing-second-picker

Fix/#323 closing second picker