Introduce SDK versions (= CKEditor versions) and make them available for download

[AnnaTomanek]

When we finally replace the existing samples available in each CKEditor distribution with the online and downloadable SDK, we will also need to create SDK versions for particular CKEditor releases so that users were able to download the SDK release applicable to the CKEditor version they are using.

These SDK builds should be available on one download page.

First idea is to make the Download SDK button link to a new http://sdk.ckeditor.com/download page that would list all available SDK versions.