z-index calculation should not stop at first element

i guess that when calculating proper z-index for the datepicker, we should find the max z-index value of parents() and not stop at first

related to #533 #170 #683 #678 #656 #665 #634 #621 #609
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Merge branch 'master' of git://github.com/fadomire/bootstrap-datepicker into fadomire-master

Conflicts:
	js/bootstrap-datepicker.js
Merge pull request #710 from fadomire/master