Renormalize line endings

As we have more people contributing to the project, it is better if we set up line ending consistency now rather than have problems in the future. The preferred way of doing this is with .gitattributes. I have renormalize the entire repo using this configuration, and the only files that have changed are in dep/decNumberICU, so fortunately we don't have any inconsistencies yet.

Works towards #769 (closed)