Now the zip_files() function is working better:

Now the zip_files() function is working better:

- More checks are done
- Non existing files are out
- Both internal an external are supported and protected
- All Petr's patchs applied
- Included support to Win32 external zip.

TODO: Create a paralel unzip_file() function
      Use this functions in all the FileManager scripts

THOUGHT: External zip seems to work always adding files to
      zip file if it exists, where PclZip resets the file.
      Perhaps a rm (or delete) should be included in the
      command chain. Analyse it!!

Merged from MOODLE_14_STABLE