title: Python numpy loadtxt fails with date time
body: <p>I am trying to use numpy loadtxt to load a csv file into an array. But it seem i can't get the date time correctly loaded.</p>
  
  <p>Below demonstrates what is happening. Did I do something wrong?</p>
  
  <pre><code>&gt;&gt;&gt; s = StringIO("05/21/2007,03:27")
  &gt;&gt;&gt; np.loadtxt(s, delimiter=",", dtype={'names':('date','time'), 'formats':('datetime64[D]', 'datetime64[m]')})
  Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/numpy/lib/npyio.py", line 796, in loadtxt
  items = [conv(val) for (conv, val) in zip(converters, vals)]
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/numpy/lib/npyio.py", line 573, in &lt;lambda&gt;
    return lambda x: int(float(x))
  ValueError: invalid literal for float(): 05/21/2007
  </code></pre>