Add details about why we use Tor Hidden Services for reporting

Related to: https://github.com/TheTorProject/ooni-probe/issues/107
Merge pull request #127 from aagbsn/feature/document_how_ooni_uses_bridges

Feature/document how ooni uses bridges
Merge pull request #126 from hellais/docs/tor-hs-motivations

Add details about why we use Tor Hidden Services for reporting
Fix reference to global_options
Fix method generatePcapFilename
Check for root once at startup