title: assign dtype with from_dict
body: <p>I have data in a python dictionary like:</p>
  
  <pre><code>data = {u'01-01-2017 22:34:43:871': [u'88.49197', u'valid'],
          u'01-01-2017 11:23:43:803': [u'88.49486', u'valid'],
          u'02-01-2017 03:11:43:898': [u'88.49773', u'valid'],
          u'01-01-2017 13:54:43:819': [u'88.50205', u'valid']}
  </code></pre>
  
  <p>I can convert it to a pandas Dataframe with:</p>
  
  <pre><code>data = pandas.DataFrame.from_dict(data, orient='index')
  </code></pre>
  
  <p>but I am not able to use the dtype parameter of <code>from_dict</code>. I would convert the index to a <code>datetime</code> of similar first column to float and third to string.</p>
  
  <p>I have tried:</p>
  
  <pre><code>pandas.DataFrame.from_dict((data.values()[0]), orient='index', 
                                                 dtype={0: 'float', 1:'str'})
  </code></pre>
  
  <p>but it doesn't work.</p>