Adding a few missing package.json fields

Adding a few missing fields to the package.json so it should more or less pass http://package-json-validator.com/ validator.

Current output:
```json
{
  "valid": true,
  "warnings": [
    "Missing recommended field: contributors"
  ],
  "recommendations": [
    "Missing optional field: engines"
  ]
}
```

Also, added the `gruntplugin` keyword so the module should show up in the http://gruntjs.com/plugins directory.
Merge pull request #1 from pdehaan/patch-1

Adding a few missing package.json fields