Commit from trunk of wp.org repo
Update file-level documentation

Changes from GPL3 license to GPL2+.

References GitHub repo, including allowing updates from there.
Add banner and icon assets
Refactor class into a separate file

Stops using the half-implemented Singleton pattern for the class.
Tidy up documentation

New README and CHANGELOG files.
2.0.0

* Refactor class into a new file. Stops using half-implemented Singleton pattern.
* Update documentation.
* Add GitHub Updater support.
Merge branch 'release/2.0.0'