Remove SNI warnings, add necessary dependencies

SNI can be done quite easily with a bunch of new dependencies.
But a word of warning, all these dependencies installed easily
on my machine, a development machine with ubuntu 14.04.
I don't know if this will be true for other distributions,
or machines not being used heavily for development
How very very stupid of me. That was NOT a conditional import
Add documentation why these extras exist

I wanted to remove the pyOpenSSL dependency
as it is a requirement of ndg.httpsclient anyway.
But now I reference to the extras section of
requests that would provide the same functionality,
referencing to this extra but requiring different
packages would be confusing.
make the dns resolver configurable for v4 or v6 (or both), update docs

made the dns detector less special: it can now get configured as any other detector.

this is most useful to give either family:INET or family:INET6 options, so dyndnsc will behave less confused
as before (when it regularly updated IPs, just because of comparing 2 different kinds of IPs).

there is also a hostname:host.name.to.query.com option (defaults to hostname from first updater so
one usually does not need to give it).

refactored some code into a separate parse_detector_opts function to avoid duplication.
Merge pull request #26 from nsupdate-info/dns-af-supp

make the dns resolver configurable for v4 or v6 (or both), update docs (try 2)
Merge pull request #14 from do3cc/SNI

 Remove SNI warnings, add necessary dependencies for py<3.2