Add .gitattributes file to deal with line endings

When running `grunt check` after cloning the repo and installing dependencies the Grunt JSCS tasks `grunt jscs:assets` and `grunt jscs:grunt` fail with 33 errors `Invalid line break at filename.js` (Where filename.js is any of the 33 JavaScript files included in the Compass repo, the exception is `/grunt/log.js` as it is not included in task files list)

In my personal global `.gitconfig` I set `autocrlf = true` which has in the past dealt with the vast majority of newline (LF/CR/CRLF) issues with GitHub repo's and Windows though in this case with Compass it does not.

If I change the `"validateLineBreaks": "LF",` rule in `.jscsrc` to `CRLF` then the JSCS Grunt tasks pass without error and without a `.gitattributes` file per this patch, thus I went with the `.gitattributes` patch to as I think this would be the best fix for the issue and maximum compatibility.

See also: https://help.github.com/articles/dealing-with-line-endings/