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
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.
Added composer.json
New: Testuite created