Investigate cross-browser compatibility

[bminixhofer]

Permon uses Babel to make the Javascript cross-browser compatible. However, I noticed some problems with the Request API in Safari. Some other browsers might not work as expected too.
Permon does however already work on the latest versions of major browsers so it is not vital to fix this before 2.0.0.

If you encounter any issues with cross-browser compatibility, please post them here. I will systematically test all browsers I want to support later and add a checklist for tested browsers here.