Closes #758, Replace tab icon

Switch to red Z alone. Only 16px for now, since that's what we have in the red
Z.

chrome/content/zotero/tab.xul

@@ -27,7 +27,7 @@
 	<script type="application/javascript"
 		src="chrome://global/content/contentAreaUtils.js"/>
 	<xhtml:link rel="shortcut icon"
-		href="chrome://zotero/skin/zotero-z-16px.png" style="display:none"/>
+		href="chrome://zotero/skin/zotero-new-z-16px.png" style="display:none"/>
 	<popup id="contentAreaContextMenu">
 		<menuitem id="context-undo"
 				label="&undoCmd.label;"