Enable FrameworkVersion and allow Nuget packages without references

Hi,

as my projects need to be .net version 4.5 and the android build needs a minimum android version, I've extended the xslt to allow setting the framework version and profile per platform.

In addition I fixed a bug in ProjectGenerator using vs2012 to include nuget packages and resyncing the project afterwards (nuget specs without a references element throws exception).

Any comments/extensions are welcome.

Commit log:
"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)"