make source_path in script optional, fixes #32

jedi/api.py

@@ -198,7 +198,7 @@ class Script(object):
     :param col: The column to complete in.
     :type col: int
     :param source_path: The path in the os, the current module is in.
-    :type source_path: string
+    :type source_path: string or None
     """
     def __init__(self, source, line, column, source_path):
         self.pos = line, column