.gitattributes and filter to add copyright header

The filter script appends Typhoon header when adding/modifying files, and removes standard Xcode header if found.

It seems to work for the simple cases I have tried:
- Create a new file without any kind of header.
- Create a new file with Xcode header.
- Create a new file with Typhoon header.
- Modify an existing file with Xcode header.
- Modify an existing file with Typhoon header.

It did me something weird after a reset. Status keep telling me there was changes in a file, and there weren’t changes any more.

Probably needs a change in the readme/contributing page to tell other contributors to “install” the filter. Nothing happens if the filter is not defined (not even a warning).

See #116