Added support for ipaddr6

That allows the client to parse and print IPv6 addresses.
Example from radiusclient:
Framed-IPv6-Address              = 'fd05:930b:5f29:97dd::4'
Added support for ipv6prefix

That allows the client to easily parse types like
Framed-IPv6-Prefix. Example from radiusclient:
Framed-IPv6-Prefix               = 'fd05:930b:5f29:97dd::/64'
Added RFC6911 IPv6 attributes
dictionary: added IPv6 types like ipv6addr
Merge pull request #20 from nmav/ipv6

This patch set adds support for IPv6 address and prefix parsing