feat(datepicker): add disabled and ngDisabled support

[wesleycho]

• Adds support for disabled attribute
• Adds support for ng-disabled

This implements #4059.

[Foxandxss]

I am not too fond of using disable in here because it is only meant for concrete elements. I think that ngDisabled is enough.

All things considered, not that Angular is respecting all conventions in HTML

[wesleycho]

My reasoning is that we are treating this as a control, so we should try to hook into the descriptive native to implement the behavior desired. I know disabled has a particular semantic, but I think it's ok to use it, as long as we are disabling things sensibly.

I forgot to link it, but here is the Plunker in action.

[Foxandxss]

Yeah, I think it makes sense, to have a read only datepicker. Go for it.