Update file tools versioning in VS projects.

Remove versions from imported properties files so that they can be safely imported across platform versions. Update project ToolsVersion according to VS defaults:

* VS2010: 4.0
* VS2012: 4.0
* VS2013: 12.0

More info [here](http://social.msdn.microsoft.com/Forums/vstudio/en-US/9e75f650-ebfe-4d17-a9b1-8f860997dd36/toolsversion-errors-from-msbuild-visual-studio-2012-and-2013?forum=vcgeneral).