proposal(datepicker): use native input element

[jefersonestevo]

This issue is a proposal to change the span that shows the current value from the datepicker to use a native input from the browser.

From:
<span class="md2-datepicker-input-text">{{_viewValue}}</span>

To something like:
<input [(ngModel)]="_viewValue" [required]="required" disabled class="md2-datepicker-input-text" />

With this change, we can use the native form validation from the browser on the datepicker

[leolorenzoluis]

Submit a pull request please