As part of this, introducing some new pom properties:

* core.version: version of google-cloud-core
* beta.version: version used by beta APIs (which is the project
  version plus -beta)
* storage.version: the version of google-cloud-storage (since
  several APIs depend on it)

Lastly, removing "(Alpha)" from titles in README files except
the root README file, because it's hard to keep all those
tags up to date.