Add custom headers to checks

I needed to have the possibility to add custom headers to the requests being made. 
With this pull requests `checks.json` can now have a hash containing headers:
```json
{
  "checks": [
    {
      "name": "check with custom headers",
      "url": "http://example.com",
      "frequency": 5,
      "headers": {"HeaderFoo": "foo", "HeaderBar": "bar"},
      "method": "GET",
      "data": null
    }
  ]
}
```
Additionally the Header field could be added as a prompt on the rake task: `bundle exec rake checks:add`