Added the minDate and maxDate options on the datePicker

First of all: this is my first pull request EVER. 
So, sorry.

Anyway, with some lines of code in datepicker.js, especially in the "template" function, i had add the possibility to the people to add the min and max date.

I added the class "uk-datepicker-date-disabled" on the days that are not selectable. So i edit the datepicker.less file too.

I hope I can help. I love this amazing Framework, i use it for all of my works and if I can give you helps I'll be happy as a thirteen guy watching Scarlett Johansson nude pics.

There are many of this little features that I have done, and I'll be happy to show you more if you like the way I code. 

Peace.
Paolo