Some tests and documentation for the systemd notification feature.
Also fixes some corner case issues detected while writing the tests.

As it is only required to run the tests move pytest-runner from
setup_requires to tests_require as suggested by @jonathanunderwood
on #115.

These changes introduce support for sdnotify allowing sshuttle to notify
systemd when it finishes connecting to the server and installing
firewall rules, and is ready to tunnel requests.

This should fix an issue introduced in #117 where when no subnets are
given via file (-s file) the variable is None instead of an empty list
and the concatenation with the subnets given as positional parameters
fails.

By just splitting at spaces, multi-word arguments are torn apart even if
quoted. In case of custom ssh-cmd, this makes it practically impossible
to set certian options through `ssh -o`.
shlex splits arguments like a shell and e.g. respects quotes.

This should fix #116. Handling this while still having the positional
arguments and -s both write to the same list turned out to be more
complicated than it's worth so each writes to their own variable and we
merge them at the end.

requirements.rst: Fix mistakes

Closes #111.

IPv6 support for OSX and BSDs

Adds IPv6 support for OpenBSD and OSX.

AF_INET is the same constant on Linux and BSD but AF_INET6
is different. As the client and server can be running on
different platforms we can not just set the socket family
to what comes in the wire.

We were always excluding 127.0.0.1/8 but sshuttle might be listening on
other IP, e.g., ::1 for IPv6 or any other defined with -l

Should fix issue #104.

Add <forward_subnets> to divert rule in OpenBSD

Fixes bug where all traffic routed to loopback would end up being diverted to the same port.

A possible implementation for the change requested in #94, so that seed
hosts can be used without auto hosts. In this scenario only the
specified hosts (or ips) will be looked up (or rev looked up).

Otherwise the output can vary and confuse our attempts to parse it.

Fixes: 93

We shouldn't come up with a fatal error because of a ENETUNREACH when
trying to contact the DNS server. Although this error shouldn't happen
either.

Fixes #89.

Sometime ago I was in python mode and incorrectly indented a line of the
shell script with spaces instead of tabs. Shame on me. This should bring
things back to their natural order.

Previously the sshuttle shell script would pass the python to use as the
first argument of the command. The new run script no longer does this.
Instead we can obtain the python being used via sys.executable.
Fixes #88.