Update .gitignore

This answer on StackOverflow http://stackoverflow.com/questions/2143956/gitignore-for-visual-studio-projects-and-solutions#answer-8855705 suggests using .gitignore files from https://github.com/github/gitignore

The .gitignore for this repository has been updated with the Visual Studio file from:
https://github.com/github/gitignore/blob/master/VisualStudio.gitignore