Stop using "key" for component references

[mairatma]

Some of you might have already used this functionality from Metal.js, of specifying a key to reference sub component instances.

I've recently noticed that using key for this this can cause problems, since that's also used internally by incremental dom for optimizing rendering paths (similar to React's key for virtual dom). Using it for component references could cause problems since we're merging two functionalities here.

React uses ref instead for this, so I thought that this would be the best name for us too, since the usage is basically the same.

I've just finished this change and released the latest version, so I'm creating this issue more for documenting the change, in case anyone has issues with key suddenly not working anymore. Just replace it with ref and all will be fine :)

I've already updated the site with this too, so take a look if you're curious.