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)
Refactored new FrameworkVersions to support defaults.
Include TargetFrameworkVersion/Profile in both project and configuration.

This is required to work around MonoDevelop/Xamarin.
Merge pull request #3 from hach-que/JoschaMetze-master

Enable FrameworkVersion and allow Nuget packages without references