Allow queries to be executed asynchronously

[cjrowe]

At present FreesoundClient only allows queries to be executed synchronously, blocking thread execution. This may be acceptable for 'small' calls (such as retrieving details of a single sound, pack, or user), but not really for longer-running ones (such as downloading sounds/packs).

Unirest has built in support for async execution of queries (returning Future objects) - the library should make use of this functionality.

[cjrowe]

Need to consider how best to do this. Made a start by using Unirest.asJsonAsync() method (which returns a Future<HttpResponse>), but as there is nothing for the library to do until that completes, we just end up blocking immediately on the Future.get() method.

There may be benefit in doing this anyway from a Unirest point of view, but I had intended building a method on FreesoundClient that returned a Future<Response> to a submitted query, ensuring users of the library are not waiting on queries completing.