Refs #3942
 * Zeitgeist is the foundation of our vision for an Open, Extendable Analytics Platform: Piwik 2.
 * This is the default theme for Piwik 2 from which other themes automatically inherit.
 * Plugins do not need a PluginName.php class anymore: plugin can be just a theme! (a json file + less/css/templates overloads)
 * Displaying themes Zeitgeist and PleineLune in the Themes listing
Refs #3942
 * Enabling one theme a time: automatically deactivates another theme if it was enabled (except Zeitgeist which is always on)
Merge remote-tracking branch 'origin/master'