Client certificate support and example

[eyazrgeotab]

Support for client side certificates.

[aaront]

Hey @eyazrgeotab, I think the example file is unneeded for this.

Also, can you implement custom certs for the aiohttp async API (in py3/api_async.py)? I try to keep it a feature-parity interface with the syncronous API. Support for certificates can be added like: https://docs.aiohttp.org/en/stable/client_advanced.html#ssl-control-for-tcp-sockets

Thanks!

[eyazrgeotab]

@aaront I removed the example and added certificate support to the async API as well.