Functional interface cleanup

[lhazlewood]

Current 0.10.0-SNAPSHOT base (master) has a few of duplicate(ish) interfaces in the io.jsonwebtoken.io package that could probably be consolidated with a single functional interface. This would have to be an internal io.jsonwebtoken.lang interface since we can't utilize JDK 8 until 1.0 final.

[lhazlewood]

Leaving for 1.0 to utilize JDK 8 interfaces.

[UkrainianCitizen]

@lhazlewood How close are you to 1.0?