Improved handling of output to terminal and files.

New module file_io which takes care of this.  All output should go
through file_io.write, which encodes it using the appropriate
character encoding.  For output to a temporary file, use "with
file_io.tmpfile('name')".

Moved WorblehatException to new module exc, so all modules can access
it without importing worblehat.

cli/exc.py

@@ -0,0 +1,3 @@
+class WorblehatException(Exception):
+    def __init__(self, msg):
+        Exception.__init__(self, msg)