Bug/inconsistent result when indexing DataFrame by YY-MM-DD
```
rng = date_range('2012-01-01', periods=1000, freq='12H')
vals = randn(1000)
df = DataFrame(vals, index=rng)
```

`df.ix['2012']` and `df['2012-05']` (selection by year or year and month) work as expected. However:

```
In [314]: df.ix['2012-01-01']
Out[314]: 
0    0.107819
Name: 2012-01-01 00:00:00
```

The output from indexing by year/month/day is unexpected - we should receive either all values matching the day or an error (if indexing by day has not been implemented), instead we get only the first match.
