Turn DWaveSampler into context manager

[randomir]

Blocked by #77.

[arcondello]

One thing we could do is only keep the thread pool open if inside a context. So

sampler = DWaveSampler()
sampler.sample_ising(...)
sampler.sample_ising(...)

would be equivalent (from a cloud-client perspective) to

with DWaveSampler() as sampler:
    sampler.sample_ising(...)
with DWaveSampler() as sampler:
    sampler.sample_ising(...)

where as

with DWaveSampler() as sampler:
    sampler.sample_ising(...)
    sampler.sample_ising(...)

would only open one thread pool.

However this would impact performance pretty significantly in the case of multiple calls outside of a context manager. Probably better to do the singleton threadpool mentioned by @randomir in #171 (comment).

[randomir]

@arcondello, I think we'll soon have singleton threadpools, as they are related to dwavesystems/dwave-cloud-client#326.