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.

I agree. However, I wasn't sure whether that's a known property. This is how `Microsoft.Common.CurrentVersion.targets` defines it:

```xml
    <!-- During DesignTime Builds, skip project reference build as Design time is only queueing information.-->
    <BuildProjectReferences Condition="'$(BuildProjectReferences)' == '' and '$(DesignTimeBuild)' == 'true'">false</BuildProjectReferences>
    <!-- By default we will build (and if applicable, clean) all project references. But this can be used to disable that-->
    <BuildProjectReferences Condition="'$(BuildProjectReferences)' == ''">true</BuildProjectReferences>
```