Add this extension to the Zikula Extension Library

#### Hi @craigh!

You requested to add this extension to the [Zikula Extension Library](http://zikula.org/library/) :star:. You're just two clicks away from there:
1. Merge this PR!
2. Add a new Tag as version 8.0.2 (either using the git command line, your favourite git client or the
[GitHub online interface](https://github.com/craigh/PostCalendar/releases/new))!
3. You're done! After your extension is published, you can edit your vendor information (such as `logo`, `title`, `homepage` and `email` [at zikula.org](http://zikula.org/library/edit-vendor-information).

In case something doesn't work as expected, feel free to open an issue in the [Extension Library repository](https://github.com/craigh/ExtensionLibrary/issues/new)!