Minor: Added .gitnore, .md extension and SPDX IDs

- Added .gitignore
- Changed file-extension of README.markdown to .md which is more common
  than .markdown.
- Switched to SPDX license identifiers
- @author tag more common link brackets
Version bump v1.1.17

The readme makes more clear now that this class is just not stable to
use compared with the Dropbox API.