Adds demo for DrawerView

[amlcurran]

This adds a simple "DrawerView" or bottom sheet behaviour to iOS projects.

The integration is relatively simple, and relies on constraints making it a bit more robust.

To integrate it you must:

• Add a "peek view" which is the view always shown (red in the movie)
• Add a "content view" which can be shown when dragged (green; this is automatically scrollable)
• Set the constraint which will be animatable.

The API includes a basic delegate which can be used to add a dimming background view, as shown.

[florianmski]

@amlcurran 👍 or 👎 ?