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 trick for opaque iterator is to include the last id, instead of the offseet as well, so any `next` will generate the following query: `select * from charges where id > last_id offset x` assuming that `id` is incremented integer for example. In this case query would be more efficient than with offset, as it would use DB index more effectively.