Do not remove plugin on cleaning empty placeholder

This should fix #98

The embed plugin insert a div before the placeholder:

```javascript
add : function ($placeholder) {
    var formHtml = '<div class="medium-editor-toolbar...';
    $(formHtml).appendTo($placeholder.prev());
```

The fix for Firefox that cleanup empty placeholder (related to #28) has to check if this `div` exists before removing it.

I've made it kind of generic by checking for a div element instead of a class.

I was thinking of a better way to tell the *placeholder cleaner* to not remove the parent: instead of checking that a `div` has join the party, we could add a `data-` attribute to the parent (the `mediumInsert-` div) when adding the div (in the `add` method of the plugin).

```javascript
$(formHtml).appendTo($placeholder.prev());
$placeholder.parent().data('plugin-inserted', 'embed');
```

And then, in the `placeholder cleaner` we can do that to check if we have to remove the parent :

```javascript
if (typeof parent.data('plugin-inserted') == "undefined") {
```

It can be apply to the plugin embed and table. Which option do you prefer? Should I keep the *ugly* fix in the 0.x branch and create the one with the data solution in the 1.x branch?