Issue17

Pull request for #17 .  Also reworked GenerateNuSpec and ReadPackagesConfig tasks to use NuGet classes instead of XDocument.
Is there no overload that takes a TextWriter? Then you could simply do:

```CSharp
var oldNewSource = "";
using (var stringWriter = new StringWriter())
{
     newManifiest.Save(stringWriter)
     oldNewSource  = stringWriter.ToString();
}
```