fix(datepicker): min date, max date today on initial month/year display[issue#1050]

This pull request about the initial month/year display of the datepicker for [issue#1050](https://github.com/angular-ui/bootstrap/issues/1050) .
We add logic when no model value is assigned  and the currentDate is less than **minDate**  or  more than  **maxDate** date. 
    if currentDate is less than minDate , the initial selection will be **minDate** .
    if currentDate is more than maxDate , the initial selection will be **maxDate**.
    if currentDate is in the min and max range , the initial selection will be **today**.
Also if model value is assigned ,  the initial selection will be the model value