Fix NET_4_0 tests build by correcting accessibility of properties/classes...

...that should be public according to MSDN, but were internal:

http://msdn.microsoft.com/en-us/library/microsoft.build.evaluation.project.targets(v=vs.100).aspx
http://msdn.microsoft.com/en-us/library/microsoft.build.execution.projectinstance.targets(v=vs.100).aspx
http://msdn.microsoft.com/en-us/library/microsoft.build.execution.projecttargetinstance(v=vs.100).aspx

@atsushieno I noticed you changed the .Targets properties in Project.cs and ProjectInstance.cs to internal for 4.0 with 221184f74786145fcea1bd9014c2958f7de78417, was this intentional or by accident?