title: Resample to DatetimeIndex raises an error
body: <p>I created a DatetimeIndex and I want to resample the data with that index. When I do that I get an exception:</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/local/lib/python2.7/dist-packages/pandas-0.8.1-py2.7-linux-i686.egg/pandas/core/generic.py", line 188, in resample
  limit=limit, base=base)
  
    File "/usr/local/lib/python2.7/dist-packages/pandas-0.8.1-py2.7-linux-i686.egg/pandas/tseries/resample.py", line 41, in __init__
      self.freq = to_offset(freq)
  
    File "/usr/local/lib/python2.7/dist-packages/pandas-0.8.1-py2.7-linux-i686.egg/pandas/tseries/frequencies.py", line 392, in to_offset
  raise ValueError("Could not evaluate %s" % freqstr)
  ValueError: Could not evaluate &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
  
  [2012-03-02 09:00:00, ..., 2012-03-02 15:00:00]
  Length: 73, Freq: 5T, Timezone: None
  </code></pre>
  
  <p>It seems that TimeGrouper.resample is supposed to handle DatetimeIndex but a call to to_offset in the TimeGrouper init does not. But I could be missing something here.</p>
  
  <p>Any idea how to resample by an index? 
  Or perhaps a workaround - All I'm trying to do is sample by some frequency (say, 1Min) but with start / end time that do not necessarily have data points in the original time series (I'm trying to have a bunch of time series sampled from 8am to 4pm, but some only have values from 9:30am, some from 10am etc.)</p>