Read proxy url from environment

[nschenkels]

These changes check if HTTPS_PROXY or HTTP_PROXY are set in the environment (in this order).

If so, the RESTClientObject class creates its pool_manager using urllib3.ProxyManager() instead of urllib3.PoolManager() and passes the proxy url taken from the environment variable.

[jglassenberg]

We're going to need some time to evaluate this code change. I can't provide an ETA, but it is on our list to review as we continue to update our SDKs.