Replace KeyboardEvent.key 'Dead*' with 'Dead' (FireFox)

What the title says.

http://www.w3.org/TR/DOM-Level-3-Events-key/#keys-composition
Those were all the keys I had on my keyboard, but you're definitely right it seems, I found that the URL I initially used to create most of that list (in the comment) has been moved and substancially updated:
https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent.key

It doesn't actually list them for Windows which is odd, but it does list quite a few more of them. Anyway, I updated the list with all 16 of them:

```
   raw     gz Compared to master @ 35764c5ffb14ac80dc05f64d9b1b234660784775
   +469   +156 build/react-with-addons.js
  +310   +116 build/react-with-addons.min.js
  +469   +152 build/react.js
  +310   +108 build/react.min.js
```

Probably not worth it, but...

```
      if (key.substr(0, 4) === 'Dead') {
        return 'Dead';
      } else {
        return key;
      }
```
```
  raw     gz Compared to master @ 35764c5ffb14ac80dc05f64d9b1b234660784775
  +152    +59 build/react-with-addons.js
   +29    +22 build/react-with-addons.min.js
  +152    +52 build/react.js
   +29    +20 build/react.min.js
```

... what's your opinion?