Modified project and property files to support changing the target platform toolset

Changes were made to the project/property files to support building in VS2013 but targeting another version of the VC runtime, e.g. VS2010.  This is controlled through the project properties GUI in Visual Studio for the CefSharp.Core project.

![platform-toolset](https://cloud.githubusercontent.com/assets/3039061/3592349/9b11c378-0c70-11e4-91d4-190cc3f7ffa0.png)

If no platform toolset is set it will use the version that matches the current version of Visual Studio.