Normalize line-endings

I noticed that the source files contain mixed line endings. I think it's a good idea to use normalized line endings for a cross-platform project like this.

Using the directions from https://help.github.com/articles/dealing-with-line-endings I added .gitattributes, and committed the normalized line-endings.

You can verify that there are only white-space changes with:

```git show -b 4e18597```

-b makes git show ignore white-space changes, and thus the command will show no diff.