Add option for interval-based adjustment to catch programmatic editing

Currently, if your application dynamically alters the contents of the editor with JavaScript, the plugin does *not* detect this change and fails to adjust height accordingly. 

This is because wysihtml5 uses an iframe with a `<body contenteditable="true">`.  Unlike `<input>` tags, there is no `change` event that gets fired for `contenteditable` elements.  Although there is currently a W3C spec for an `input` event, support at this time seems to be limited.  

The other way to ensure that  height gets adjusted consistently is to rely on a timer/ `setInterval()` system.  I have made changes to the plugin (which my team is using in production now) which allow you to use a timer instead of the default onkeyup, onkeydown, event-driven method.  

Can now be used like so:

```js
$(editor.composer.iframe).wysihtml5_size_matters({ useTimer: true });
.
.
.
$(editor.composer.iframe).wysihtml5_size_matters("stopTimer");
```

Default behavior is unchanged.