feat(mdpDatePicker): ng-required support

[marcomafessolli]

Supports ng-required to disable and enable required validation

[isaacfi]

Hi,

I did this yesterday, If you accept me as contributor I would integrate this functionality.

Regards.

[dpoetzsch]

I integrated this fix in my fork https://github.com/dpoetzsch/mdPickers. I will try to create a bower/npm release in the next time so others can profit from the advanced state of the fork.