date picker: support string representation of date as a model

[jayeshanandani]

Actual Behavior:

• What is the issue? * The date picker invalidates the valid ISO 8601 formatted string. It should allow to parseDate in mdLocaleProvider.
• What is the expected behavior? The parseDate function should fire prior to date picker throwing the error for string type or inbuilt support for valid ISO 8601 string format.
  This has been already raised earlier: md-datepicker: support string representation of date as a model #6253

Angular Versions: *

• Angular Version: 1.5.x
• Angular Material Version: 1.1.1

Additional Information:

• Browser Type: * Chrome
• Browser Version: * Latest
• OS: * MAC , El Captain
• Stack Traces:

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