Add Visual Studio 2015 support

Redirect VS2015 to use VS2013 binaries and toolset in CefSharp.props.

Added .vs folder to .gitignore, as it contains the .suo files and
intellisense database used by VS2015. See
http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/6079923-store-project-related-information-in-vs-folder-to
for more details.