Enhance .htaccess security rules

1. Deny access to all files not containing a . (dot) to block access to different README, ChangeLog, etc. files of various skins and plugins.

I opted for the FilesMatch rule because i felt that this might carry less of a performance penalty in comparison to mod_rewrite. Feel free to update the RewriteRule instead

2. Do not check case for default README/INSTALL/LICENSE/etc. files.
(This could also be enhanced to deny access to these files in *all* directories and there are sometimes different file names in use, e.g. ./plugins/managesieve/Changelog)