Change versions from "Master" to 0.5

This makes the element compatible with other published Polymer elements

Patch core-overlay to handle multiple backdrops.

When there are multiple core-overlays open at the same time, sometimes one of the backdrops will not close with its overlay dialog. This can leave an invisible element over the entire window, making it seem like the screen is frozen (documented here: googlearchive#37).

This patches a fix by @WrinklyNinja, and we can refer to this release from our bower.json to pull the patched code into our app.