Prevent image load requests when country is null

When src is null it still makes an invalid (404) request to load an image for null country.

Polymer 1.x and 2.x hybrid element

-) Implemented Polymer hybrid mode to support Polymer 1.x and 2.x
-) Updated README.MD to include an interactive demo

Polymer 1.0 release

v0.12.0

Update README.md