warn if IE8 is in compatibility mode

[jonhester]

Fixes #3315

[jimfb]

Looks good. Two minor changes.

1. We should use the warning module instead of console.debug. For example usage, you can search our code base for 'warning' (https://github.com/facebook/react/search?utf8=%E2%9C%93&q=warning). The warning module takes in two arguments (the invariant condition and the message to display).
2. We should wrap the call to warning in a if(__DEV__) block, as we do elsewhere in the code base (see same link).

[jonhester]

@JSFB, thanks for the feedback. I changed it to use the warning module. I did not wrap it in an if(__DEV__) block, since it is already inside the one starting on line 102.

react/src/browser/ui/React.js

Line 102 in 65c4aea

if (__DEV__) {

.

[jimfb]

Great, looks good to me! @spicyj, any feedback?

We are in a code freeze for the next week-ish, so we'll prep this for merge once the code freeze ends.