Add async timeout callback support to Java binding

[mbarger99]

Follow-on to #26. We made the updates to support a new async request timeout without breaking backwards compatibility for C++ and Python by providing a default implementation for the new callback method. For Java though, we use an interface to define the callback methods. Java 8 adds support for providing a default impl in an interface, but we need to maintain support for Java 6 and 7 for the MATLAB versions we use. So adding Java support here will break backwards compatibility as it will require all classes that implement GravityRequestor to implement this new method.

We will make this change when we have other pending changes that will also break backwards compatibility.