Add VS2013 solution and project files

This pull request adds Visual Studio 2013 solution and project files. The 2013 project files are set to use the VS2013 runtime (v120). It also adds sdf and opensdf to the .gitignore.

Benefits: 
* Can use latest VS2013 development environment for compiling libusbx
* Can use latest VC++ runtime, including better C99 support

Costs:
* Slight increase in maintenance costs for keeping all of the project files in sync.