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)
You can assume that account ids are already unique, so you don't need an extra id for simplicity