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.
Merge pull request #34 from hakre/master

Update to 1.1.17