Small fixes of repository setup.

* Remove executable bits except for executable files, in our repository
  only .sh files should be executable.
* Use Git submodule fir libi0 instead of symlink, because not every
  developer would like to use cod, so the repository should work
  correctly if directly using Git. Submodule is a feature of Git to
  include external repositories. For more information, run
      git submodule --help
* Add .gitattributes for automatic line ending handling. I found some
  files (e.g., paralloc_writer_reader.c) have CRLF line endings even in
  working copy on Linux, this is because (I guess) some developers work
  on Windows.
* Add .gitignore.