Fixed VCProject not dirty when setting global properties

Added a little hack to ensure projects are really marked dirty when setting
the global properties SolutionConfiguration, SolutionPlatform. Before
the Microsoft.Build.Evalutation.Project was marked dirty but the VCProject
was still not marked dirty or considered up-to date anyway. Adding a
dummy property and removing it immediately is enough to ensure
the VCProject is marked is no more up-to date
Moved VS2013 specific code from MainSite.cs to MainSite.VS12.cs
Moved single project configuration inside the method ConfigureCollection
Reworked ordering of async tasks

SetVCProjectsConfigurationProperties must wait for
SetVCProjectsConfigurationProperties to know if the project is
dirty or not because of collection modification.
Better understanding of async/await pattern
Fixed possible deadlock when releasing lock with ReleaseAsync()
Removed hardcoded assembly HintPath