intersection between DatetimeIndex does not perform element-wise intersection
When I have two dataframes, df1 and df2, and df1 is missing an index of df2, performing the intersection only seems to compute matching slices -- but doesn't take into account missing inner-elements.  For example, below, see that `2010-07-14 00:00:00` is missing from df1, but in df2.  The resulting index intersection, however, contains that index.  

``` python
>>> df1.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2010-05-07 00:00:00, ..., 2012-06-29 00:00:00]
Length: 542, Freq: None, Timezone: None

>>> for x in df1.index[40:47]:
...     print x
...     
... 
2010-07-06 00:00:00
2010-07-07 00:00:00
2010-07-08 00:00:00
2010-07-09 00:00:00
2010-07-12 00:00:00
2010-07-13 00:00:00
2010-07-15 00:00:00

>>> df2.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-07-14 00:00:00, ..., 2012-06-29 00:00:00]
Length: 3010, Freq: None, Timezone: None

>>> for x in df2.index[2510:2517]:
...     print x
...     
... 
2010-07-09 00:00:00
2010-07-12 00:00:00
2010-07-13 00:00:00
2010-07-14 00:00:00
2010-07-15 00:00:00
2010-07-16 00:00:00
2010-07-19 00:00:00

>>> x = df1.index.intersection(df2.index)
>>> for i in x[40:48]:
...     print i
... 
2010-07-06 00:00:00
2010-07-07 00:00:00
2010-07-08 00:00:00
2010-07-09 00:00:00
2010-07-12 00:00:00
2010-07-13 00:00:00
2010-07-14 00:00:00
2010-07-15 00:00:00
```
