Imported 5.x branch from SVN development repository.

- #153179 Patched for Drupal 5.x compatibility, corrected front page handling, and added expiration time to the Boost page footer (kudos to Alexander I. Grafov, Jacob Peddicord and Justin Miller).
- #103884 Updated the Boost .htaccess file from Drupal 5.x-dev; also added HTTP headers and special-case mod_rewrite rules for correctly handling caching of the front page (kudos to Axel).
- Imported the default .htaccess file from Drupal 5.x-dev.
- Updated credits listing in README.txt.
- Updated INSTALL.txt instructions to reflect Drupal 5.x.