email-gravatar: fix html syntax issues

* make ampersand a html entity
* add required alt attribute
* add required img end tag

filters/email-gravatar.lua

@@ -15,7 +15,7 @@ function filter_open(email, page)
 end
 
 function filter_close()
-	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&d=retro' style='height:10pt;width:10pt'> " .. buffer)
+	html("<img src='//www.gravatar.com/avatar/" .. md5 .. "?s=13&amp;d=retro' style='height:10pt;width:10pt' alt='Gravatar' /> " .. buffer)
 end
 
 function filter_write(str)