Version 2.0.0

While there is not that much new stuff added, I nontheless increased the
major version, as it's no longer fully compatible with 1.x:

- PHP 7.3 instead of 5.3
- Stricter type checks in some places
- ste now lives in the r7r\ste namespace

Some other things that I worked on since the last release (see the git
log for more details):

- More tests! We're also using phpUnit for testing now instead of our
  own thing.
- Conststent and more modern code formatting (enforced by phpcsfixer in
  a git pre-commit hook)
- Lots of other code quality improvements
- Some bug fixes