Fixed file_force module link on drupal.org in README.md and some other changes

adult referrer added;
extend known MIME types;
after upgrade from D6 to D7 uploaded in D6 images stay in there old path;
added htc extention to static served files;
restrict direct access to pot files;
disallow access to .bzr, .git, .hg, .svn, .cvs directories;
some other code comments correction.