Release 2.6.0

Changes

Refactorings for reducing code duplication.
Major refactorings to limit the JAX-RS / Jersey exposure of the code to a minimum of classes. This is a preparation for possible alternative implementations (see issue #14 ).
Freed the Station class from Jackson annotations (replacing with a mixin class).
Migrate from JMockit to Mockito since JMockit is conflicting in Java 21.
Building with JDK 21 now (still targeting JDK 8) since Mockito is needing this.
Migrate JUnit 4 to JUnit 5. Welcome to 2016.