KeyError on slicing with datetime or pandas.Timestamp
related #6066 (on Float64Index too)

The 'KeyError' on slicing was discussed multiple times but I'm still not sure if the issue below is a bug or just my misunderstanding. 

I'm experiencing 'KeyError' from time to time when I try to slice my dataframes with datetime or Timestamp objects, however slicing with strings works perfectly. I was unable to construct synthetic example with pandas.date_range, so I needed to upload the piece of real data where the issue appears:
https://www.dropbox.com/s/ibzbwqs35tiydyc/tmp.h5

```
df = pd.read_hdf('tmp.h5', 'data')
print df.head()
```

When I try to slice it with the pandas.Timestamp objects it results in 'KeyError':

```
tt1 = pd.Timestamp('2006-11-16 18:30:00')
tt2 = pd.Timestamp('2006-11-16 18:40:00')
print df.ix[tt1:tt2].head()
```

Same for datetime objects:

```
tt1 = pd.Timestamp('2006-11-16 18:30:00').to_datetime()
tt2 = pd.Timestamp('2006-11-16 18:40:00').to_datetime()
print df.ix[tt1:tt2].head()
```

However this slicing works perfectly:

```
tt1 = pd.Timestamp('2006-11-16 18:30:00').strftime('%Y-%m-%d %H:%M:%S')
tt2 = pd.Timestamp('2006-11-16 18:40:00').strftime('%Y-%m-%d %H:%M:%S')
print df[tt1:tt2].head()
```

Numpy version 1.8.0
Pandas version 0.13.0
