avoiding `None` filename and line number in step descriptions. It's an attempt…

avoiding `None` filename and line number in step descriptions. It's an attempt to close #204, which seems to happen in windows

lettuce/core.py

@@ -157,8 +157,10 @@ class StepDescription(object):
         self.file = filename
         if self.file:
             self.file = fs.relpath(self.file)
+        else:
+            self.file = "unknown file"
 
-        self.line = line
+        self.line = line or 0
 
 
 class ScenarioDescription(object):