Support Ion to JSON

[clacrone-github]

Often we have scenarios where input comes as JSON, converted to ION to do some business requirements, append data, etc, and then needs to be converted back to JSON. IonJava and others support the capability to translate ION to JSON. Add this extension for Python please.

[tgregg]

Note to anyone considering implementing this feature: the JSON down-conversion rules that should be followed are listed in the cookbook: http://amzn.github.io/ion-docs/guides/cookbook.html#down-converting-to-json