WIP emojis in TextEditor and misc fixes to TextEditor

There are still a few issues to fix:
1) If you select some text and insert a character, the selection isn't replaced by the character (this should be easy to fix)
2) Newlines might cause problems
3) A placeholder image (Mario Bros) is being used instead of the real Emojis
4) On Firefox OS, for some reason, the *word-break: break-all;* rule isn't being respected (http://jsfiddle.net/zz9wn15j/1/)

A problem related to newlines that I'm aware of can be reproduced following these steps:
- Write 'a' (content 'a', innerHTML 'a')
- Press return (content 'a\n', innerHTML 'a< br >< br >')
- Write 'b' (content 'a\nb', innerHTML 'a< br >b< br >')
- Press backspace (content 'a\n', innerHTML 'a< br >') <- Here in *content* we have a newline, but we aren't showing it in the div

(*content* is the variable holding the TextEditor content)
Nit: I know you're just copying this code, but elsewhere you use this construction:

```
if (this.oninputCallback) {
  this.oninputCallback();
}
```

It'd be good to make this one consistent.