Add frameworkAssemblies node to project.json to add gac/reference assemblies

[davidfowl]

Today we mix these in the same section which causes several issues including #619. We're going to separate these out into a new section (consistent with the nuspec):

{
  "frameworks": {
    "net45": {
      "frameworkAssemblies": {
        "System.Xml": "4.0.0.0"
      }
    }
  }
}

These won't be restored using kpm restore and they'll be put in the frameworkAssemblies section when generating the nupkg.

[Praburaj]

@davidfowl @ChengTian I added something like this to my project.json

"frameworks": {
        "aspnet50": {
            "frameworkAssemblies": {
                "System.Xml": "4.0.0.0",
                "System.Data": "4.0.0.0"
            }
        }
    }

While doing a kpm restore i see this error. Should the version be still "" or it is an actual GAC assembly version?

Resolving complete, 7927ms elapsed
Unable to locate System.Xml >= 4.0.0.0
Unable to locate System.Data >= 4.0.0.0
Installing Microsoft.AspNet.SignalR.Client 2.1.1
Installing Newtonsoft.Json 5.0.1
Installing Xunit.KRunner 1.0.0-beta1-10371
Installing Microsoft.Framework.TestAdapter 1.0.0
Installing xunit.execution 2.0.0-aspnet-alpha4-10007
Installing xunit.core 2.0.0-aspnet-alpha4-10007
Installing xunit.abstractions 2.0.0-aspnet-alpha4-10007
Installing xunit.assert 2.0.0-aspnet-alpha4-10007
Restore complete, 8526ms elapsed

[davidfowl]

Looks like a bug somewhere. I'll take a look