Add CachedRender wrapper

[jgaskins]

This keeps the component from being injected with virtual-DOM logic. It can simply be its own Ruby object. The wrapper acts as an adapter between the two.

There are two main benefits of this:

• You can now have a @type instance variable. Previously, if you defined one in your component, it would prevent the virtual-DOM engine from caching it.
• You can define a key method on your component to optimize diffing/patching when child nodes are moved around or deleted. Previously, you either had to wrap the component in a container node with a key property (slower because it would have to be diffed without hints) or set an @key instance variable (completely unintuitive in Ruby).

The downside of this is that it allocates an additional object for each cached component on every render, but I haven't seen this cause any negative effect on realistic performance. Even in my contrived app that renders (among other things) 1000 cached list items, performance improved (from 9ms to 4.5ms per render) with no noticeable difference in memory usage.

Posting a PR in case there's something I may be missing. Render caching is pretty important for Clearwater so I don't want to screw it up. :-)