Changed namespace to official namespace assigned by XEP-0363 (http://xmpp.org/extensions/xep-0363.html)

 * Content-type and content-length are checked for every URL in MUCs and regular chats
 * Plugin config now have a setting for max image download size (which is checked to content-length)
 * If content-type match and content-length is smaller then setting the image will be downloaded and displayed
 * If image can be loaded, the URL is no longer displayed (except as a tooltip on the image)
 * Image is clickable (opens browser with URL)
 * Images are displayed using XHTML
 * URL is displayed until image is displayed
Fixes #102, #105