* Make pre_release.sh script work with GNU
The `-i` sed flag is for doing in-place updates of the file. On some
systems it requires a parameter, on other (GNU) the parameter is
optional, but should be "glued" to the flag (`-i.bak` for example).
That's is problematic, so just emulate the in-place updates with
copies and removals.
* Update the version string in sdk with the pre-release script
This was a missing step, which is why we still have a version string
"0.2.3" while in reality it should be "0.3.0". Hopefully it will be
something to fix in "0.3.1" or what the next release version will be.
* Ensure clean git state when doing a release
The script does `git add .` which adds everything in the tree, even
the untracked files. Make sure that there are none of those.
* Update License header for all source files
- Add Apache 2.0 header to source files that did not have one.
- Update all existing headers dated to 2019 to be 2020
- Remove comma from License header to comply with the Apache 2.0
guidelines.
* Update Copyright notice
Use the standard Copyright notices outlined by the
[CNCF](https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices)
