.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.

One gotcha: you will not have the correct header after you checkout that file again. The changes happen from the working directory to the staging area, but do not persist in the working directory until a checkout of that file happens.

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