Fix for currency/username null returns, implementation of various missing fields

* Fixed currency and username on a payment request returning null, per issue https://github.com/apruve/apruve-java/issues/5
* Implemented various Apruve API fields that had not previously been implemented, per issue https://github.com/apruve/apruve-java/issues/6
* Fixed setting of currency, (part of issue https://github.com/apruve/apruve-java/issues/6 )