Improved cache headers and various bug fixes

• New: Added HTTP cache headers for cached files
• New: Added configuration from _ss_environment.php file constants
• New: Added quiet mode to rebuild cache task
• Bugfix: Make sure URLs that get cached are always part of allPagesToCache
• Bugfix: Config not loading properly for RsyncMultiHostPublisher
• Bugfix: publishPages must use Live stage
• Bugfix: Don't ignore $cacheBaseName for cache files