Inheritance refactor

Changed class communication structure to inheritance as per issue #1. An example use of this library is shown in ExampleClass (also shown below) which simply prints out poses. Note that onPeriodic must be called in the event loop.
```c++
template <class BaseClass = PoseGestures<>, class PoseClass = PoseGestures<>::Pose>
class ExampleClass : public BaseClass {
 public:
  void onPose(myo::Myo* myo, PoseClass pose) {
    std::cout << pose << std::endl;
  } 
};

int main() {
  try {
    myo::Hub hub("com.voidingwarranties.myo-intelligesture");
    myo::Myo* myo = hub.waitForMyo(10000);
    if (!myo) {
      throw std::runtime_error("Unable to find a Myo!");
    }
    ExampleClass<> listener;
    hub.addListener(&listener);

    // Event loop.
    while (true) {
      hub.run(1000 / 20);
      listener.onPeriodic(myo);
    }
  } catch (const std::exception& ex) {
    std::cerr << "Error: " << ex.what() << std::endl;
    return 1;
  }
  return 0;
}
```