Updating package.json

Should now pass http://package-json-validator.com/ with minimal warnings:
```json
{
  "valid": true,
  "warnings": [
    "Missing recommended field: keywords",
    "Missing recommended field: contributors"
  ],
  "recommendations": [
    "Missing optional field: engines"
  ]
}
```
Merge pull request #70 from pdehaan/patch-1

Updating package.json