title: Pandas resample function not working on DateTimeIndex
body: <p>I have a data frame called <code>aus</code> that looks like this:</p>
  
  <pre><code>+--------------+-------------+
  |              | link clicks |
  +--------------+-------------+
  | created_time |             |
  | 2015-07-20   |        8600 |
  | 2015-07-21   |       11567 |
  | 2015-07-22   |        1809 |
  | 2015-07-23   |        7032 |
  | 2015-07-26   |       23704 |
  +--------------+-------------+
  </code></pre>
  
  <p>I make the index a DateTimeIndex like so:
  <code>aus.index = pd.to_datetime(aus.index)</code></p>
  
  <p>Then I run a check like this: <code>type(aus.index)</code> and the given output is <code>pandas.tseries.index.DatetimeIndex</code></p>
  
  <p>Then when I try to resample the index into weeks like so 
  <code>aus.index = aus.resample('w', how='sum', axis=1)</code> I am met with the following error:</p>
  
  <pre><code>---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-310-3268a3f46a19&gt; in &lt;module&gt;()
  ----&gt; 1 aus.index = aus.resample('w', how='sum', axis=1)
  
  /usr/local/lib/python2.7/site-packages/pandas/core/generic.pyc in resample(self, rule, how, axis, fill_method, closed, label, convention, kind, loffset, limit, base)
     3264                               fill_method=fill_method, convention=convention,
     3265                               limit=limit, base=base)
  -&gt; 3266         return sampler.resample(self).__finalize__(self)
     3267 
     3268     def first(self, offset):
  
  /usr/local/lib/python2.7/site-packages/pandas/tseries/resample.pyc in resample(self, obj)
      100             return self.obj
      101         else:  # pragma: no cover
  --&gt; 102             raise TypeError('Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex')
      103 
      104         rs_axis = rs._get_axis(self.axis)
  
  TypeError: Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex
  </code></pre>
  
  <p>My type check earlier says I have the proper index, but the resample function doesn't thinks so.
  Any thoughts?</p>