Documentation updated, fixes for bug #195/196 (previous commit comment was wrong):

Documentation updated, fixes for bug #195/196 (previous commit comment was wrong):
- Fixes for missing function SQL_ESCAPE() while in installation phase and setting
  language (e.g. call install.php?mx_lang=de), see ticket #196
- Added code to check for double-initialization of cache instance, see ticket #195
- Updated documentation, may be missleading, see ticket #198
- TODOs.txt updated