Make line endings CRLF by default, and for all files

Depending on the operating system, the line ending used by default varies. Adding a .gitattributes file ensures that git will make the line endings will be constant across systems. Most of the files here seem to use CRLF file endings (probably from Windows), so I've made that the setting to be used across all files. If you want more information on git and line endings, see here: https://help.github.com/articles/dealing-with-line-endings/