Fix line endings and add .gitattributes file

This adds the .gitattributes file from https://github.com/Danimoth/gitattributes/blob/master/CSharp.gitattributes and normalizes line ending in git to LF using the method described in https://help.github.com/articles/dealing-with-line-endings (under "Re-normalizing a repo").

I'm pretty new to dealing with line endings in git but I believe this is a normal approach for handling this issue and should help avoid any line ending issues in the future.