Simplify and reduce checks (delegate to tar)

- Since the tar command is run by the user and not the class or the root
  it can only access and archive files readable by the user.  That means
  we don't need to check about these files.  We can let linux do all the
  permission checking for us.  Under the same spirit we do not need to
  worry about absolute and relative paths including ".." (CAUTION: this is
  only regarding files to be submitted and not the assignment path under
  TURNIN or the class name)
  As a result check_symlinks is no longer needed.
- Introduce strict checks for file-paths for the assignment

Should resolve issues #2, #4, #7, #10.