Adding scripts section to project.json

Launched by kpm commands via shell command processor

Example usage 
```
  "scripts": {
    "prerestore": "echo prerestore",
    "postrestore": "echo postrestore",

    "prepare": "echo preparing %project:Name% %project:Version%",

    "prebuild": "echo prebuild",
    "postbuild": "echo postbuild",

    "prepack": "echo prepack",
    "postpack": "echo postpack"
  }
```

`prepare` is executed at the very end of `kpm restore` and at the very start of `kpm pack`
 

Property not field.