fix(datepicker) disabledDates sorting fix and docs update

Fixes sorting for feature implemented at https://github.com/mgcrea/angular-strap/commit/bb5038064a681842539984d5626ea550ff3c4c8e. Removes sorting requirement of date ranges. There were also no unit tests for disabledDates so I created some.

disabledDates wasn't listed in the docs so I added that there as well.