js复制功能兼容IE,firefox

测试过IE7和firefox下没问题，chrome下失效

function buttonCopyCode(txt ){
	if (window.clipboardData) {
		window.clipboardData.clearData();
		window.clipboardData.setData("Text", txt);
	} else if (navigator.userAgent.indexOf("Opera") != -1) {
		window.location = txt;
	} else if (window.netscape) {
		try {
			netscape.security.PrivilegeManager
					.enablePrivilege("UniversalXPConnect");
		} catch (e) {
			alert("你使用的FireFox浏览器,复制功能被浏览器拒绝！\n请在浏览器地址栏输入“about:config”并回车。\n然后将“signed.applets.codebase_principal_support”双击，设置为“true”");
			return;
		}
		var clip = Components.classes['@mozilla.org/widget/clipboard;1']
				.createInstance(Components.interfaces.nsIClipboard);
		if (!clip)
			return;
		var trans = Components.classes['@mozilla.org/widget/transferable;1']
				.createInstance(Components.interfaces.nsITransferable);
		if (!trans)
			return;
		trans.addDataFlavor('text/unicode');
		var str = new Object();
		var len = new Object();
		var str = Components.classes["@mozilla.org/supports-string;1"]
				.createInstance(Components.interfaces.nsISupportsString);
		var copytext = txt;
		str.data = copytext;
		trans.setTransferData("text/unicode", str, copytext.length * 2);
		var clipid = Components.interfaces.nsIClipboard;
		if (!clip)
			return false;
		clip.setData(trans, null, clipid.kGlobalClipboard);
	}
	else{
		alert("你的浏览器不支持一键复制功能");
		return;
	}
	alert("复制成功");
}