Setup JSON application

This should be a complete implementation of the JSON interface described in the README. I did make a few minor changes to the API spec along the way, namely:

* Page limits must be positive integers less than 1000
* Account IDs cannot contain special characters (i.e. not alphanumeric or `_`)
* I dropped the timezone from the datetime format (at least until I can get the `%z` datetime directive working)
I was thinking for the right name for the class, I guess it's `Iterator` by nature, what do you think?