- add LICENSE files for Snappy and Metrics core
- update Copyright year
- copy airlift reference to the NOTICE file of the binary packages
- copy the Java 8 disclaimer from branch-3.5

Author: Enrico Olivelli <eolivelli@apache.org>

Reviewers: Andor Molnar <anmolnar@apache.org>

Closes #1196 from eolivelli/fix/license-stuff