Init multiple editors without html option: textarea contents could get overridden

Actually I have 3 textareas, with different ids and different texts inside. However when I instanciate wym editor every textarea content gets filled with the first textarea content. It does work well with 1.0b3 but not with b5
The reason that this doesn't seem possible to me is that in [the beginning of trident's `.initIframe`](https://github.com/mightyiam/wymeditor/blob/4b813aef10fd79a09ba79f0f54404717415966df/src/wymeditor/editor/trident.js#L16) there's this:

```
    if (this._doc.designMode !== "On") {
        this._doc.designMode = "On";
        // Initializing designMode triggers the load event again, thus
        // triggering this method again. We can short-circuit this run and do
        // all of the work in the next trigger
        return false;
    }
```

So in the case of Trident, we can't rely on the return value of this method. Or, at least, I don't know how we can—because it turns asynchronous.

So, the only place where we should set `.iframeInitialized` to `true` is near the end of each engine's `.initIframe`.

What do you think?