some VS project template fixes and added templates for grpc_csharp_ext

build.json

templates/vsprojects/vs2013/gpr.vcxproj.filters.template

templates/vsprojects/vs2013/grpc.vcxproj.filters.template

...sprojects/vs2013/grpc_csharp_ext.vcxproj.filters.template

templates/vsprojects/vs2013/grpc_csharp_ext.vcxproj.template

.../vsprojects/vs2013/grpc_unsecure.vcxproj.filters.template

templates/vsprojects/vs2013/grpc_unsecure.vcxproj.template

templates/vsprojects/vs2013/vcxproj.filters_defs.include

vsprojects/vs2013/grpc.sln

vsprojects/vs2013/grpc_csharp_ext.vcxproj

vsprojects/vs2013/grpc_csharp_ext.vcxproj.filters

vsprojects/vs2013/grpc_unsecure.vcxproj