Add Client.get_proxies() (and similar accessors)

[HamletDuFromage]

Hello,
I ran into a scenario where I needed to access the proxies I was using on a particular instance of a client, and I couldn't do it in an easy way. I saw in #842 that you were not looking to add a way to change proxies while the client is open, but I would like having _proxy as a member of the client class, and a function get_proxies() defined like this

def get_proxies(self):
    return self._proxies

Thank you for the module though, great stuff :)