Fix for EntityExtended ignoring stop/once values in animSettings

`stop` and `once` values in EntityExtended.animSettings were being
ignored.

Specifically, this fix changes...
* *core/entity.js* -> `createAnim` - pass settings.stop/once to addAnim
* *core/entity.js* -> `addAnim` - added once param
* *core/animation.js* -> `playFromStart` - check for once in settings
* *core/animation.js* -> `rewind` - check for once in settings

I added issue https://github.com/collinhover/impactplusplus/issues/30 to represent the issue that this should fix.