Add copyright checker to tox

+ Only looks at files edited since HEAD~.
+ Only checks files of whitelisted extensions.
+ Distinguishes missing and outdated copyrights.
+ Any line with "Copyright" substring can work.
+ Many copyright lines are tolerated, if at least one has correct year.
+ Voting.

Change-Id: I2cda0459cb191eeec7aada69c508973568039d87
Signed-off-by: Vratko Polak <vrpolak@cisco.com>