Add reference to parent instance for component instances

@chenglou I'm pretty sure you want something like this for your event overhaul right? (to get rid of ID hacking)

After my short discussion with @sebmarkbage in #1922, I realized that I could implement a compromise of #1570. Where instead of removing `data-reactid` entirely, I can shorten it to just `data-react` (with no value) until the refactor is done and we have had a chance to evaluate and decide on the most preferable behavior.

The advantage of the compromise is that it will be more isolated and will work identically to the current lazy implementation. So server-generated markup should be considerably smaller (compression would virtually remove any `data-react` overhead), we also gain some of the performance benefit of the shorter markup.

This is probably a hot topic, but I believe this is inevitable if we want to get rid of ID hacking and concatenated IDs (which I'm positive we do want to get rid of).

One thing that I'm unsure of is whether it should be `parent` or `parentNode` (ignore composite components), as composite components should be irrelevant for our internal purposes.