introduce file version control

The Craedl SDK now supports Craedl's newly introduced file version control feature.

Changes:

• Directory.create_file(...) is unchanged in the SDK and automatically adds a new version of the file if the file exists and has changed.

• File.download(...) now takes an optional version_index, to allow you to choose to download a particular version. The index comes from the chronological ordering listed in the File.versions attribute. Leaving version_index unspecified downloads the version that is currently active.