Fix for issue #13

See issue #13 for details. The summary is:

* We don't want to build in case we're performing a design time build as we are expected to not produce any assets.

* We also don't want to build the references in cases where we build inside the IDE. The reason is that Visual Studio already built our dependencies. Doing it again can regress performance. However, the real issue is that it impacts correctness as this can result in building the same project simultaneously from different projects.

I've changed the build to respect the `$(BuildingInsideVisualStudio)` and `$(DesignTimeBuild) ` properties.

Hehe... I thought the same. However, `Microsoft.Common.CurrentVersion.targets` does it the same way:


```xml
<!--
  ============================================================
                                      CleanReferencedProjects

  Call Clean target on all Referenced Projects.
  ============================================================
  -->
<Target
    Name="CleanReferencedProjects"
    DependsOnTargets="AssignProjectConfiguration; _SplitProjectReferencesByFileExistence">

  <!--
      When building the project directly from the command-line, clean those referenced projects
      that exist on disk.  For IDE builds and command-line .SLN builds, the solution build manager
      takes care of this.
      -->
  <MSBuild
      Projects="@(_MSBuildProjectReferenceExistent)"
      Targets="Clean"
      Properties="%(_MSBuildProjectReferenceExistent.SetConfiguration); %(_MSBuildProjectReferenceExistent.SetPlatform)"
      BuildInParallel="$(BuildInParallel)"
      Condition="'$(BuildingInsideVisualStudio)' != 'true' and '$(BuildProjectReferences)' == 'true' and '@(_MSBuildProjectReferenceExistent)' != ''"
      ContinueOnError="$(ContinueOnError)"
      RemoveProperties="%(_MSBuildProjectReferenceExistent.GlobalPropertiesToRemove)"/>

</Target>
```