Implemented Okpay client using ScalaXB.

Due to problems with the repository name change, this PR replace to PR #85

All comments was addressed on PR #85, 
Please, remember:

- Tested with real calls and real tokens, it works.
- Code covered by unit testing.
- The implementation returns a `Future` but the ScalaXB client is synchronous, in the future, when the ScalaXB 2.0 will be released, we must migrate this service in order to make it asynchronous.