Enable FrameworkVersion and allow Nuget packages without references

Framework version (e.g. for .net 4.5 or android version) can be set in
the Properties element:
<FrameworkVersions>
      <Platform Name="Windows">

<Version>v4.0</Version>
        <Profile>Client</Profile>

</Platform>
      <Platform Name="Android">

<Version>v4.2</Version>
      </Platform>
    </FrameworkVersions>

Nuget packages without the reference element in the nuspec throw
exception while generating the project. ProjectGenerator has been
changed to adhere to nuspec (add all libs as reference)