Improved resilience of libresilient

The project still has rather “alpha” quality, there’s plenty of fixes that need to be done.

Improve general error handling
Improve handling of some non-essential APIs not being implemented in the browser (Safari and postMessage(), for example)
Cleanup some old code, removing old assumptions
Improve plugin configuration (for example: enable use of the same plugin multiple times with different config)
Unit tests for the service worker, plugins and for libresilient itself