Store references to nodes that use the "ref" attribute

[mairatma]

This is a feature that has been requested before. It's basically to allow using ref inside regular dom elements, instead of only for component instances. If a ref is given to an element, its reference should be stored in the refs variable of the owner component. For example:

// Can be accessed as "this.refs.myElement"
<div ref="myElement"></div>

Note that component instances should still be accessible inside this.components like before (for backwards compatibility), but not element instances (since they're not components). Both should be accessible via this.refs. In a future major version we can get rid of this.components (since it covers only one of the use cases due to the name being too specific).

[mairatma]

This has been added in f8ee3b9.