XAFORUM-118 : Remove custom styling for buttons

Although Glyphicon is bundled by default with Bootstrap, we don't encourage it's use. 
We have IconThemes, see http://extensions.xwiki.org/xwiki/bin/view/Extension/Icon+Theme+Application and http://jira.xwiki.org/browse/XE-1389 for an usage example. 
Using IconThemes means that the icon will change depending with the active set, be that FontAwesome, Silk or a custom one.