Add wrapper element around shortcodes

[soderlind]

The generated HTML looks like this:

<div class="readoffline-shortcode">
	<a class="pdf" href="http://dev.local/read-offline/992/mammoth.pdf" title="Download mammoth.pdf">Download PDF</a>
</div>
<div class="readoffline-shortcode">
	<a class="print" href="http://dev.local/read-offline/992/mammoth.print" title="Print mammoth." onclick="read_offline_print_me();return false;">Print</a>
</div>

It should have a wrapper like this:

<div class="readoffline">
	<div class="readoffline-shortcode">
		<a class="pdf" href="http://dev.local/read-offline/992/mammoth.pdf" title="Download mammoth.pdf">Download PDF</a>
	</div>
	<div class="readoffline-shortcode">
		<a class="print" href="http://dev.local/read-offline/992/mammoth.print" title="Print mammoth." onclick="read_offline_print_me();return false;">Print</a>
	</div>
</div>