datepicker: automatic positioning

[fxck]

it should open up when there's not enough space at the bottom, otherwise it's gonna be completely unaccessible

[kara]

@fxck This component is still on a feature branch (WIP), so it's likely @mmalerba simply hasn't implemented fallback positions yet.

[crisbeto]

Reassigning as discussed with @mmalerba. Also to continue the discussion from #4627, I've looked into it a bit and there are a couple of issues:

1. We haven't set up the fallback positions for the datepicker.
2. Even if we had set up the fallback position, the ConnectedPositionStrategy measures the element too early and won't determine the proper positioning.