Preliminary IPv6 support
Bug in nksip_parse:transport/1
First working version of IPv6

- included basic test
- pending to test registrations, dns, proxies
FEATURES.md update
Doc updates
Tokenizer update

Moved into its own file
New nksip_lib:to_host/2 function

Functionality extracted from nksip_lib:to_binary/1
Minor correction
nksip_uac_lib:make/5 rework
Updated get_listening/3 to support IPv6
New option local_host6 for IPv6
Automatic detection of IPs only uses ethX and en0 interfaces
Correction in fork_test
Correction in invite_test
New ipv6_test finished
Doc updates