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)
The primary key here isn't required for uniqueness but it is used in the one-to-many relationship with charges and for the paging interface. Without it I would have to use the consumer facing account ID for the foreign key in the charges table and I think that would be a lot less efficient to store. Would you mind if I kept it?