Non-blocking handlers for artifact upload events

[kevstigneev]

Expected result:

• On a "master" Nexus instance plugin's code receives events on artifact upload.
• The receiver gets aritfact coordinates and the target repository.
• Uploads to proxy repositories are ignored.
• The event is processed as an asynchronous task.

[kevstigneev]

Done in scope of PR #9 and #13.