Fix encoding to UTF-8

Partly cherry-pick commit miicha/tabutils@ec86cf06a9b1bfe3001e64bea41eac1ed67a8765

Conflicts:
	chrome/content/tabutils.js
Translate some comments

Cherry-pick commit miicha/tabutils@96f712a1d6bf6fe7f2e27bc5669f4edd393b43a4
Fix supported Firefox versions

The original addon has been broken since Firefox 31 so this fork is only
for Firefox 31 and later. The original addon works fine with older versions and
should be used there.