Allow specification of proxy when creating a client

[tiberiuvs]

We should be able to make API calls from any of the clients through a proxy.

[kelvie-pstg]

Until this is done, the easy workaround would be to set the proxy environment variables as mentioned here:

https://stackoverflow.com/questions/31639742/how-to-pass-all-pythons-traffics-through-a-http-proxy/31641300

[JanosPesti]

in the pypureclient\pure1\configuration.py
self.proxy = None
is hardcoded, I had to add "import os" and change "None" to "os.environ['http_proxy']" in configuration.py for it to work.

[sdodsley]

Under investigation