LOG4PHP-200: Improved superglobal access in converters

Removed the current, slightly hackish access to superglobal variables,
i.e. ${$this->name}. Instead added a function in each sub-class which
returns the contents of the required superglobal. This fixes the issue and
replaces hacky accessing with more vanilla code.

Signed-off-by: Ivan Habunek <ihabunek@apache.org>