Added support for load_map_from_string in BaseWMSFactory.loadXML method (2)

Hi,
This pull request adds possibility to load mapnik XML configuration from string using load_map_from_string function. There is a check if 'xmlfile' is existing file (os.path.isfile) - if not 'load_map_from_string' will be used.
This is second pull request after #26 - earlier I've add wrong checkig - sorry for that.

Regards
Piotr Pociask