Move QR code as a plugin

[nodiscc]

The QR code should be optional (as well as archive.org and readityourself integration), even if it's enabled by default (should it?).

Once #52 is tested and merged, we can use the plugin system to move the QR code to tpl/plugins/qrcode/. This plugin will require 2 parts:

• The HTML elment <div style="position:relative;display:inline;"><a href="http://qrfree.kaywa.com/?l=1&s=8&d=urlencodedlink" onclick="showQrCode(this); return false;" class="qrcode" data-permalink="url of link"><img src="images/qrcode.png" width="13" height="13" title="QR-Code"></a></div> - can be injected in the linklist using LINKLIST_PLUGINS
• The script itself <script language="JavaScript"> // Remove any displayed QR-Code function remove_qrcode() [...] can be injected in the page footer using a new FOOTER_PLUGINS option

Optional QR code requested at sebsauvage#171

[nodiscc]

This is part of #164. Please help reviewing the Pull Request, taking ideas in #14 and #106 into account so we can anticipate what's needed for a useful plugin system.