Add support for nuspec property "developmentDependency" #3

enhancement
Not sure whether this intentional but it causes the following behavior:

1. I've created a library and added a package reference to JSON.NET
2. I've opened the `packages.config` and added `developmentDependency="true"` to the `package` element.
3. I've created a NuProj project and added a reference to the class library.

The resulting NuGet package will not have a package dependency on JSON.NET but now directly includes `Newtonsoft.Json.dll`, side by side to the class library.

Is that really the correct behavior? I would have expected that the `Newtonsoft.Json.dll` wouldn't be packaged up. Maybe my understanding of development dependencies is wrong but it seems like this would be useful for cases like code gen where the dependency is only used during design time but isn't required for runtime. In that case, I don't think it would make sense to package up the files coming from development dependencies.

Thoughts?