title: Deserialize nested fields in marshmallow
body: <p>I'm consuming an API that returns something like:</p>
  
  <pre><code>{'name': 'foo', 'start': {'date': '2016-06-19', 'time': '18:00'}}
  </code></pre>
  
  <p>And I want to desearialize it with marshmallow to get only the name and the start date, so the desired result would be the following:</p>
  
  <pre><code>{'name': 'foo', 'date': '2016-06-19'}
  </code></pre>
  
  <p>But I haven't found any way to get the date, this what I have tried:</p>
  
  <pre><code>from marshmallow import Schema, fields, pprint
  
  event = {'name': 'foo', 'start': {'date': '2016-06-19', 'time': '18:00'}}
  class EventSchema(Schema):
      name = fields.Str()
      date = fields.Str(load_from='start.date')
  
  
  schema = EventSchema()
  result = schema.load(event)
  pprint(result.data)
  </code></pre>