datepicker: add `exportAs` in directives and components

[ocarreterom]

Bug, feature request, or proposal:

Proposal.

What is the expected behavior?

Nice to use:

<md-input-container>
  <input mdInput [mdDatepicker]="startDatepicker" #startDatepickerInput="mdDatepickerInput">
</md-input-container>
<md-datepicker #startDatepicker></md-datepicker>
<md-input-container>
  <input mdInput [mdDatepicker]="endDatepicker" [min]="startDatepickerInput.value">
</md-input-container>
<md-datepicker #endDatepicker></md-datepicker>

What is the current behavior?

Currently the use of NgModel or FormControl is required.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

angular/material: 2.0.0-beta.5-05dbb90

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