fix mixins ability to handle concatenated properties when the base does not have a value for it, add an override specifier for mixins so they can continue to override all properties on the base (default) or set it to false to defer to any properties already set on the base, add instance and global hasMixin methods to programatically determine if an instance of kind already has the mixin applied to it.

Enyo-DCO-1.1-Signed-off-by: Cole Davis <cole.davis@lge.com>
Merge pull request #264 from clinuz/master

fix and feature update for mixins

Reviewed-By: Ben Combee (ben.combee@lge.com)