ssl: support client certificates

[mathstuf]

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Tested with Davical+nginx locally.

[mathstuf]

Addresses #182.

[untitaker]

Thanks for this patch!

• I think auth_cert is a more fitting name, because then all auth-related stuff starts with auth_. We also support JSON lists in the config, so only one param is needed, and the user can use both PEM and key/cert formats (see cert param in http://www.python-requests.org/en/latest/api/#requests.request)
• Please add an example to the docs you added, for both formats.
• Please also add this parameter to HttpStorage. There's also a shared docstring in there, HTTP_STORAGE_PARAMETERS. Add some docs for that parameter there too.

[mathstuf]

Updated.

[untitaker]

Awesome, thanks!