Execute callables within Gumshoe managed context

[lwoodson]

Addresses issue #26 to give callback interfaces to be executed within a Gumshoe managed context. The context will start, execute the callable, finish the context (or fail it if exception) then return result (or propagate exception).

Two interfaces are supported -- the standard lib Callable interface when you care about a return value and the Gumshoe provided VoidCallable interface when you don't care about a return value.

This works w/anonymous classes:

context.start(new VoidCallable() {
    public void call() {
        // do something
    };
});

and java 8 lambda syntax:

context.start(() -> {
    // do something
});

[lewisren]

💯