Cond IE8

@zpao I did a quick test just to see what would happen if we singled out IE8 and made it possible to strip its from the final build. I'm pretty confident that no other old browser got caught in the cross-fire, but would obviously have to validate (not that I recommend taking this PR as-is).

Current master size:
```
649770 133520 build/react-with-addons.js
135443  36996 build/react-with-addons.min.js
591846 121396 build/react.js
125593  34400 build/react.min.js
```

IE8 compatible build (can be made smaller):

```
  +569    +55 build/react-with-addons.js
  +452    +77 build/react-with-addons.min.js
  +569    +59 build/react.js
  +452    +79 build/react.min.js
```

IE9+ compatible build (isIE8 is constant false):

```
  -273    -36 build/react-with-addons.js
 -1338   -407 build/react-with-addons.min.js
  -273    -38 build/react.js
 -1338   -405 build/react.min.js
```

So it stripped away slight more than 1% (from master), mostly courtesy of 3 large functions being stripped out of the event system I assume.