Add function to set the time elements of a date to the start of a day.

Passed in min and max date options might have there time set to user´s local time zone.
This caused problems when comparing these dates.

Replaces the use of setHours and fixes #41
Merge pull request #42 from rikkert/issue41

Add function to set the time elements of a date to the start of a day.
Merge fix for #41 into jQuery plugin.
Add Bower package definition, fix #29
Add UMD pattern to jQuery plugin.