Anbindung BibSonomy API

[zesaro]

Mandatory checks

Closes #158

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

ToDo:

• Untersuchung und Nutzung von Version puma-4.2.3 (aktuell v3.9.2 im Einsatz)

[koppor]

Good start 👍