Add .as_posix() call when reading configuration files

Ticket #29, correction suggested by @GillesOrban
Should fix potential issues when package is used on Windows. Tested on
Mac OS X without any particular issue.

pyzelda/zelda.py

@@ -105,7 +105,7 @@ def __init__(self, instrument, **kwargs):
         config = ConfigParser.ConfigParser()
 
         try:
-            config.read(str(configfile))
+            config.read(str(configfile.as_posix()))
 
             # mask physical parameters
             self._Fratio = kwargs.get('mask_Fratio', float(config.get('mask', 'Fratio')))