support ipv6 tunnels

[g0djan]

Hi @pahaz , it will fix #157 and #194 issues. We want to use it in our project and I will be glad to fix any troubles with this PR asap.

[coveralls]

Coverage decreased (-0.3%) to 91.761% when pulling c23785f on g0djan:ipv6 into e198ce5 on pahaz:master.

[pahaz]

Checklist:

• write https://github.com/pahaz/sshtunnel/blob/master/changelog.rst#changelog
• up package version https://github.com/pahaz/sshtunnel/blob/master/sshtunnel.py#L39
• add self to https://github.com/pahaz/sshtunnel/blob/master/changelog.rst#contributors
• pull from master

[g0djan]

Checked that it works with IPv6 only hosts and still works with IPv4 hosts.

[pahaz]

Ok. lets ship it