Normalizes line endings

Normalizes line endings and adds a gitattributes file to prevent recurrence of the issue.

Since the diffs of doing this are large, you might want to just do it yourself.

I added the stock `.gitattributes` file generated by VS.  Team Explorer -> Settings -> Git Settings -> Repository Settings.

Then ran the following to correct the existing files:

git rm -r --cached . 
git add .
git add -f FluentAssertions.WinRT.Specs/WinRT.Specs_TemporaryKey.pfx

The last line is needed because it is otherwise ignored by `.gitignore`