long waits on keyboard interrupt

Using dwave.system.samplers.DWaveSampler, I occasionally want to stop the process I'm running with a KeyboardInterrupt. This often takes upwards of 10 seconds... which seems a little excessive. Here's the traceback, when the interrupt finally gets caught.

``` File "/usr/local/lib/python2.7/dist-packages/dimod/core/sampler.py", line 189, in sample
response.change_vartype(Vartype.SPIN, data_vector_offsets={'energy': offset})
File "/usr/local/lib/python2.7/dist-packages/dimod/decorators.py", line 173, in new_f
return f(*final_args, **final_kwargs)
File "/usr/local/lib/python2.7/dist-packages/dimod/response.py", line 803, in change_vartype
self.data_vectors[key] += data_vector_offsets[key]
File "/usr/local/lib/python2.7/dist-packages/dimod/response.py", line 211, in data_vectors
self._resolve_futures(**self._futures)
File "/usr/local/lib/python2.7/dist-packages/dimod/response.py", line 588, in _resolve_futures
for future in as_completed(futures):
File "/usr/local/lib/python2.7/dist-packages/dwave/cloud/computation.py", line 311, in as_completed
done, not_done = Future.wait_multiple(not_done, min_done=1, timeout=timeout)
File "/usr/local/lib/python2.7/dist-packages/dwave/cloud/computation.py", line 257, in wait_multiple
event.wait(wait_time)
File "/usr/lib/python2.7/threading.py", line 614, in wait
self.__cond.wait(timeout)
File "/usr/lib/python2.7/threading.py", line 340, in wait
waiter.acquire()
KeyboardInterrupt
```