Implement interfaces for easier mocking

[rcywongaa]

Since actionlib is frequently used as part of a larger system, it would be nice if commonly used classes, e.g. SimpleActionClient, SimpleActionServer implemented an interface so that they can be easily mocked when unit testing the rest of the system. It doesn't seem like a big / breaking change and if the core devs are agreeable, I can prolly implement it and submit a PR.