Add .gitattributes and normalize line endings

This change adds a .gitattributes file that normalizes line endings in the repository for all *.vim files.  Line endings will be converted to the host system format on checkout.  The line endings for all affected files are also normalized in this commit.  This allows this plugin to work correctly on OS X and Linux, e.g. with Mono and Unity3D C# toolchains.

See also: [Dealing with line endings](https://help.github.com/articles/dealing-with-line-endings).