Harmonize line-endings

This changes the line endings of a bunch of files. All text-files now end with line feed (the unix-variant). Additionally we also add a `.gitattributes` file which tells git to use an appropriate line ending on checkout and writing back to the repository, regardless of the operating system.

Please review.

/cc @chrismayer It'd be extra nice if you could do one test of this, as I know that you sometimes work from a windows machine.