You have to specify SpecificVersion=true for the Microsoft.VisualStudio.Extension DLLs or else you can't build from VS2010 on a machine with both VS2010 and VS2012 -- it tries to use the VS2012 DLLs and fails miserably.
Adding my own assembly info
Made it build so it works in VS2010 and VS2012.   Seems to be crashy though.