[addons] Renamed update() directives to commands
Add clearer invariants to composite spec mixing
Carry ajax error checking from step #13 forward to other ajax steps

The following steps also have an ajax function, but the 'error:' param
is gone after #13:
#14
#17
#19
#20
This may be superfluous, but it helped me find an error with something I
was doing - Namely, in my .json file, I had single line javascript
comments ("//") that I copied from the tutorial. I couldn't find the
issue on later steps, but was able to see my issue when the error
handler complained about an unexpected "/" in my file in step #13.
[Docs] Rendering null/false and getDOMNode.
Test for correct handling of "download" attribute

These tests currently fail as there is no special treatment for this kind of
attribute.

Related: GH-1337
Support minimizable, non-boolean attributes

Fixes GH-1337
Rename CAN_BE_MINIMIZED to HAS_BOOLEANISH_VALUE
Combine valid value checks
Test more falsey values
Update tutorial.md
Improve error for nested render calls

In the future we could consider wrapping the entire public API (renderComponent, setProps, setState, etc) in this check but this should do for now.

Test Plan: grunt test
Restore CSSTransitionGroup tests

Not sure what happened here.

Test Plan: grunt test
Merge pull request #1405 from spicyj/css-tests

Restore CSSTransitionGroup tests
Make code spacing more consistent

I'm a little surprised we don't have lint rules for this.
Merge pull request #1284 from plievone/directives-2-commands

[addons] Renamed update() directives to commands
meta http-equiv attribute name
Merge pull request #1386 from georgesisco/master

In the tutorial, carry ajax error checking from step 13 forward to other ajax steps
Merge pull request #1384 from chenglou/render-docs

[Docs] Rendering null/false and getDOMNode
Merge pull request #1403 from spicyj/keyword-spacing

Make code spacing more consistent
Drastically improve test performance