Add ability to pass HTTP headers to Neo4j server

  - **New:** Pass HTTP headers to Neo4j using
    `GraphDatabase::query(query, params={}, options={}, callback)` where
    `options` can contain `headers` object, e.g.

    ```javascript
    options = {
      headers: {
        'X-Request-ID': '14159265359'
      }
    }
    ```

/cc @aseemk 
Minor code style: instead of a hanging indent, indent this line just one level.