title: NuGet error "External packages cannot depend on packages that target projects" when installing SNAP 1.4
body: <p>I get the following error when trying to install SNAP 1.4 into a fresh VS2010-project using Nuget 1.3: <br/>
  "External packages cannot depend on packages that target projects"
  <br/><br/>
  Is this a problem with the SNAP Nuget package, or am I doing anything wrong?</p>
  
  <blockquote>
    <p>PM> Install-Package SNAP 
    'Castle.Core (≥ 2.5.2)' not installed. Attempting to retrieve dependency from source...
    Done. <br/>
    Install-Package : External packages cannot depend on packages that target projects. <br/>
    At line:1 char:16 <br/>
    + Install-Package &lt;&lt;&lt;&lt;  SNAP <br/>
        + CategoryInfo          : NotSpecified: (:) [Install-Package], InvalidOperationException <br/>
        + FullyQualifiedErrorId :  <br/>NuGetCmdletUnhandledException,NuGet.PowerShell.Commands.InstallPackageCommand <br/></p>
  </blockquote>