Add query parameter for only showing frozen balances
[TASK] refactor converters

- move converting logic to appropriate classes
- rename rest_to_lib_transaction_converter to rest-to-tx-converter
- refactor method signatures and working to be consistent
- add unit tests for converters
[TASK] refactor var names

- async_callback to callback where appropriate
- under_score to camelCase
- change `notifications.getNotificationHelper()` to use callback instead of the missing `next`
Merge pull request #223 from geertweening/task/refactor_converters

[TASK] refactor converters
Merge pull request #233 from ripple/request-frozen-balances

Add query parameter for only showing frozen balances