Implemented Okpay client using ScalaXB.

Due to problems with the PR #85 (my custom fork was not correctly linked to github), i have closed the other and continue the code review on this PR.

Reminder from PR #85:
- 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.