adding container option, and tests

new feature : container option

Basically adds a container option, which takes a string "css selector" as per boot strap tooltips http://getbootstrap.com/javascript/#tooltips 

(useful for me, as I'm playing with zoom and transforms in my project and want the date picker popup to have these applied without hacks)

https://github.com/eternicode/bootstrap-datepicker/issues/896

ps first time pull requesting, apologies if i'm doing something wrong