Allow data download from Poland

Current version of the code is very US focused while stooq is a Polish website. To download data from the WSE one need to provide a ticker only (such as PZU), website returns null if one add PL at the end. My small addition copes with this issue while still allowing US focus (i.e. use of ticker only approach to shares listed in US).

Author: MicBan

pandas_datareader/stooq.py

@@ -39,9 +39,11 @@ def _get_params(self, symbol, country='US'):
         symbol_parts = symbol.split(".")
         if len(symbol_parts) == 1:
             symbol = ".".join([symbol, country])
+        elif symbol_parts[1].lower() == 'pl:
+            symbol = symbol_parts[0]
         else:
             if symbol_parts[1].lower() not in ['de', 'hk', 'hu', 'jp',
-                                               'pl', 'uk', 'us']:
+                                               'uk', 'us']:
                 symbol = ".".join([symbol, 'US'])
 
         params = {