Issue8

I added Database::remove_file() function to remove a file from the database, and added files_engine::delete_file() to remove the file from libtocc's file system, and the high level (front_end) function Manager::delete_file().
plus, Unit test of the function Manager::delete_file().
Please add this algorithm to ``file_info_converter.h``. It can be use in other cases. Thanks (: