Add dataType to all $.ajax calls for consistency

Fixes https://groups.google.com/forum/#!topic/reactjs/WWA3ZqU6y4w.
Allow null return values for functions to-be-merged.
Make findComponentRoot faster with more nodeCache

This is an alternative, less-invasive, fix for #891.

Test Plan:
On http://jsbin.com/OqOJidIQ/2/edit, got timings like

[75, 56, 30, 36, 27, 27, 28, 32, 27, 27, 28, 31]

instead of the old

[75, 729, 46, 32, 28, 34, 26, 27, 27, 30, 26, 26].

I also added a counter to getID and saw it was called 3014 times instead of the old 636264. (3014 is the number of nodes (3000) plus 3 calls that happen for the initial render and 1 for each of the 11 renders after that.)
Merge pull request #884 from spicyj/tut-datatype

Add dataType to all $.ajax calls for consistency
Merge pull request #895 from spicyj/zomgperf

Make findComponentRoot faster with more nodeCache
Fix "Uncontrolled Components" documentation