Add proper SSL and IPv6 support

[Jason Gerard DeRose]

Although Microfiber can already make requests to HTTPS URLs, it doesn't verify the server cert, and it doesn't have all the flexibility we need.

So we're now going to create a proper SSLContext for HTTPS URLs and use CERT_REQUIRED for the verify_mode.

And I'm extending the *env* API to allow you to also:

 * provide a custom ca_file and/or ca_path for verifying the server cert (otherwise the openssl default will be used)

 * provide a client cert (and client key if the cert doesn't contain the key)

 * turn off host-name verification (needed for dynamic stuff with Avahi on the localnetwork)

As far as IPv6, Microfiber already supports it perfectly (as far as I can tell). But we don't have unit tests or documentation for IPv6 cases, so I'm working on that.

There is a similar bug in UserCouch, which will need to land first:

https://bugs.launchpad.net/usercouch/+bug/1050575

I'm going back and forth between these two bugs right now in order to get the design of the *env* extension right. What I'm thinking of is an optional 'ssl' sub-dictionary something like this:

env = {
    'url': 'https://127.0.0.1:6984/',
    'ssl': {
        'ca_file': <filename>,
        'ca_path': <dirname>,
        'cert_file': <filename>,
        'key_file': <filename>,
        'check_hostname': False,
    }
}

Everything in env['ssl'] is optional, although providing the 'key_file' doesn't make sense unless you also provide 'cert_file'.

Also, env['ssl'] only has any effect when env['url'] is an https:// URL.