BUG: .drop() doesn't parse strings for DatetimeIndex.
Needs to coerce:

e.g.

``` python
In [1]: import pandas.util.testing as tm

In [2]: pan = tm.makePanel()

In [3]: pan
Out[3]:
<class 'pandas.core.panel.Panel'>
Dimensions: 3 (items) x 30 (major_axis) x 4 (minor_axis)
Items axis: ItemA to ItemC
Major_axis axis: 2000-01-03 00:00:00 to 2000-02-11 00:00:00
Minor_axis axis: A to D

In [4]: pan.drop('2000-01-03', axis=1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-11e47d7c99f1> in <module>()
----> 1 pan.drop('2000-01-03', axis=1)

../pandas/core/generic.pyc in drop(self, labels, axis, level)
   1112                 new_axis = axis.drop(labels, level=level)
   1113             else:
-> 1114                 new_axis = axis.drop(labels)
   1115             dropped = self.reindex(**{ axis_name: new_axis })
   1116             try:

../pandas/core/index.pyc in drop(self, labels)
   1614         mask = indexer == -1
   1615         if mask.any():
-> 1616             raise ValueError('labels %s not contained in axis' % labels[mask])
   1617         return self.delete(indexer)
   1618

ValueError: labels ['2000-01-03'] not contained in axis
```
