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.

Most particularly on the correctness side, this shows up when VS is doing a "rebuild". NuProj will end up causing multiple build breaks being reported because it will re-delete outputs that VS just produced and that other project references that are building in parallel now expect to be there.