6.0.0: Only override page title if there is no default value (#22)

* Add silverstripe/cms to composer requirements
* Make SinglePageAdmin class abstract
* Fix reference to now namespaced interface
* Use Versioned constants for set_stage
* Use publishRecursive instead of deprecated doPublish
* Populate defaults when new page is created
* Only override page title if empty
* Remove auto-publish when single-page-admin creates a page
* Fix docblock placement