avoid utf8 error

added text factory in order to avoid utf8 error as discussed here...
http://oksoft.blogspot.in/2015/09/pandas-and-sql-queries.html

Author: shantanuo

pandasql/sqldf.py

@@ -77,6 +77,7 @@ def conn(self):
         else:
             # create the connection
             conn = self.engine.connect()
+            conn.text_factory = str
             self._init_connection(conn)
             try:
                 yield conn