[Enhancement] Ability to make Calendar HTML5 Required

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Current behavior
When adding required or required="true", the p-calendar element has the required attribute - however the input that is nested in the calendar does not. This means that the form will still submit even without filling in the field.

This is similar to a previous request for p-dropdowns (which works great): #230

Expected behavior
Ability to add required or required="true" to p-calendar and have HTML5 pick up the field as part of its validation.

What is the motivation / use case for changing the behavior?
Using simple HTML5 validation is all my application requires, this makes it difficult to require date fields without creating various work-arounds.

Angular version: 2.1.2

PrimeNG version: 1.0.0