json cannot serialize circular references about python-fire

[dbieber]

Saurabh may take this as a first task.

[saurabhkpatel]

I will submit the pull request soon for this bug.
My thoughts are something like this:

Handle extra type of exception and add the appropriate message in front of the particular key. In the below example, key's value is represented same like this.

fire: <module 'fire' from '/Users/srpatel/github/python-fire/fire/__init__.pyc'>
x:    {'y': {...}}

[dbieber]

Fixed by #49, thank you Saurabh!