Kpm wrap update existing projects instead of creating new ones in wrap folder

parent #904 
You mentioned that `properties that we add to the existing  project.json  files should not overwrite what is already there`. Here are all the cases where we don't update property values:

For an existing wrapper project.json
```json
"frameworks": {
  "net45": {
    "bin": {
      "assembly": "Path/to/assembly",
      "pdb": "Path/to/pdb"
    },
    "dependencies": {
      "NuGetDependencyA": "1.0.0",
      "ProjectDependencyB": "1.0.0"
    }
  }
}
```
1. If net45 bin/pdb path of original csproj is changed to a new path and `kpm wrap` is called again, net45 bin/pdb path in project.json above is NOT updated.
2. If net45 dependency `NuGetDependencyA` of original csproj is changed to a higher version and `kpm wrap` is called again, the version of `NuGetDependencyA` in project.json above is NOT updated. It's also true for project dependency versions.

Does it look good?