Fix allowing global installation of Firefox extension

This commit fixes an issue (#246) whereby the Firefox extension does not work if installed globally.

After some research, it is determined that the issue is caused by hardcoding of rulesets.sqlite's (the master rules database's) location. It is hardcoded as paths `["extensions", "https-everywhere@eff.org", "defaults", "rulesets.sqlite"]` from the profile directory (`"ProfD"`). Unfortunately, if the extension is not installed in the profile directory ([installed](//mike.kaply.com/2012/02/09/integrating-add-ons-into-firefox/) [globally](//developer.mozilla.org/en-US/Add-ons/Installing_extensions), for example), the database could not be loaded and therefore no requests would be converted to HTTPS.

This commit fixes the issue by improving the way the extension locate the database. The database is now included in the chrome:// namespace by modifying the chrome.manifest file, and its location is determined by converting the chrome://  URL with the `chromeToPath()` function.