IN operator and %groups% placeholder

This implements:
 - IN operator by use of `~~` 
 - the placeholder `%groups%` which is replaced by the groups of the current user

Fixes #85 

Notes:
 - test fails, some about the translations, are not my cup of tea..
 - but i got also:

```
PHPUnit_Framework_Error_Warning : implode(): Invalid arguments passed
#0 [internal function]: PHPUnit_Util_ErrorHandler::handleError(2, 'implode(): Inva...', '/home/gerrit/do...', 391, Array)
#1 /home/gerrit/dokuwiki/dokuwiki/lib/plugins/data/helper.php(391): implode('','', NULL)
```
Remarkable is that the php code `$val = implode("','", $val);` is replaced in this error message by only single quotes. 
However, running the code in the wiki is fine...

Feedback appreciated!