Add decorations directive to support L.PolylineDecorator plugin

Summary of changes:

* New directive at src/directives/decorations.js
* New setDecorations and getDecorations methods at src/services/leafletData.js
* Scaffolded tests for decorations directive at test/unit/decorationsDirectiveSpec.js.  Just has a basic test right now.
* A simple example using the decorations directive at examples/decorations-simple-example.html