Add a frameworkAssemblies tag to nuspec

```
Clients using NodaTime need a reference to System.Xml due to us supporting 
serialization, as per http://stackoverflow.com/questions/28220624/

We can indicate this in the Nuspec package file, as per
http://docs.nuget.org/create/nuspec-reference#specifying-framework-assembly-refe
rences-(gac)
```

Original issue reported on code.google.com by `jonathan.skeet` on 3 Feb 2015 at 7:05
