Give precedence to plugin.* actions over custom tasks registered by plugins
Maintain a stack of currently running plugin hooks
Make possible to set vacation start/end date and time
Synchronized localizations with Transifex
Remove 3rd party libs from our repository and define the dependencies in composer.json-dist.
Also remove the ancient utf8 lib and replace it with 'Patchwork UTF-8 for PHP'.

For direct git checkouts, copy composer.json-dist into composer.json and run
`php composer.phar install` to install the dependencies.