Update contribution guidelines and license

The contribution guidelines have been cleaned up as well as parts of the
README file. Thanks to the Bootstrap team which has written such an
excellent CONTRIBUTING.md file that I just had to copy some (a lot) from
it. Documentation is licensed under creative commons. This wasn't very
clear before, but it is and now we have a docs/LICENSE file to prove it
;)

Also add the latest fixes and features to the NEWS file.
Bump version to 0.26.3
Fix notes on release handling
Fix editing of VRF in web UI

Used the wrong input name.

Also make statistics JS conditional to prevent javascript error.

Fixes #575.
Disable Travis-CI pylint and S3 upload test

We want pylint results over time and the running of pylint together with
uploading the results to S3 was a step in that direction. Recently we
found Scrutinizer-CI which can achieve the same things and as the
Travis-CI build process is considerably lenghtened by the S3 upload, I
have now disabled that test. We will rely entirely on Scrutinizer-CI!

Part of #274.
Merge pull request #576 from plajjan/575-fix-vrf-edit

Fix editing of VRF in web UI