BUG: (GH3925) partial string selection with seconds resolution
this no longer has much to do with #3925, and is only fixing a bug

Minor revision to select on second frequency

```
In [11]: df = DataFrame(randn(5,5),columns=['open','high','low','close','volume'],index=date_range('2012-01-02 18:01:00',periods=5,tz='US/Central',freq='s'))

In [12]: df
Out[12]: 
                               open      high       low     close    volume
2012-01-02 18:01:00-06:00  0.131243  0.301542  0.128027  0.804162  1.296658
2012-01-02 18:01:01-06:00  0.341487  1.548695  0.703234  0.904201  1.422337
2012-01-02 18:01:02-06:00 -1.050453 -1.884035  1.537788 -0.821058  0.558631
2012-01-02 18:01:03-06:00  0.846885  1.045378 -0.722903 -0.613625 -0.476531
2012-01-02 18:01:04-06:00  1.186823 -0.018299 -0.513886 -1.103269 -0.311907

In [14]: df['2012-01-02 18:01:02']
Out[14]: 
                               open      high       low     close    volume
2012-01-02 18:01:02-06:00 -1.050453 -1.884035  1.537788 -0.821058  0.558631

```

this is a bit inconsistent with the to-be-deprecated arith ops broadcasting behavior is it not?
 i find it the following a bit confusing, maybe it's just me

```
In [35]: df.loc[df.index[:1]]
Out[35]:
                            open   high    low  close  volume
2012-01-02 18:01:00-06:00  0.645 -1.347 -0.257 -0.816   0.155

In [36]: df[df.index[0]]
Out[36]:
                            open   high    low  close  volume
2012-01-02 18:01:00-06:00  0.645 -1.347 -0.257 -0.816   0.155
```

is there really that much added by saving 5 characters? i think it makes it confusing because now u have 2 remember that rows are sliced if a single date is passed, but if u passed 0 that will `__getitem__(self, 0)` column lookup for the key `0` + if a slice is passed slice the rows + the closed intervals gotcha + column indexing + loc, iloc, ix, iat, at + `xs` + how all of this stuff plays with `MultiIndex`. doesn't seem like adding another thing to remember about indexing is beneficial.
 @cpcloud you are right, this is inconsisten.

Sicing with a string is fine because its treated as a slice (of course could just return 1 label)

and this is fine, but a single datetime is not ....(just like `df[0]` would be rejected in a integer index based frame)

```
In [28]: df[df.index[1]:df.index[2]]
Out[28]: 
                               open      high       low     close    volume
2012-01-02 18:01:01-06:00 -0.332505  0.705486 -0.447803 -0.134373  1.706555
2012-01-02 18:01:02-06:00 -0.590720  0.519744 -0.881321 -0.886215 -0.292298
```

Integer indexed frame

```
In [19]: df = DataFrame(randn(5,5),columns=['open','high','low','close','volume'])

In [20]: df
Out[20]: 
       open      high       low     close    volume
0  1.144082  0.842330  1.774455 -2.126466  0.139598
1  0.635929  2.139363 -0.594184  0.516679  0.348589
2  1.101361 -0.520283  2.191319 -0.072571 -0.749482
3  0.535801  0.768180  1.360995 -0.512688  2.990026
4  1.297711 -0.549184  1.457773 -1.740196  0.442782

In [21]: df[0:3]
Out[21]: 
       open      high       low     close    volume
0  1.144082  0.842330  1.774455 -2.126466  0.139598
1  0.635929  2.139363 -0.594184  0.516679  0.348589
2  1.101361 -0.520283  2.191319 -0.072571 -0.749482

In [22]: df.loc[0:3]
Out[22]: 
       open      high       low     close    volume
0  1.144082  0.842330  1.774455 -2.126466  0.139598
1  0.635929  2.139363 -0.594184  0.516679  0.348589
2  1.101361 -0.520283  2.191319 -0.072571 -0.749482
3  0.535801  0.768180  1.360995 -0.512688  2.990026

In [23]: df[0]
KeyError: u'no item named 0'

```
 is this going to be merged? i would assume no as per the other thread....close?
 do you have a problem with the new PR? (which reverses out the last change) and only fixes a bug
 oh sorry yeah it's fine! :)
 @wesm was there a reason that second resolution via string not originaly implemented?
 what is the bug here? i wasn't aware that passing a date string with any resolution was allowed unless that was in the column index...
 partial string indexing

try

df['2012']
df['2012-01']

etc (generate dates of string frequencies)
 ah i c thanks.
 http://pandas.pydata.org/pandas-docs/dev/timeseries.html#datetimeindex

I think the docs need expansion here in any event
 hm so should this eventually be for all dt strings? current not working for usecs
 guess should add that too

though this is for string slicing

not sure how useful that would be
(as u would prob want to construct using datetime ranges)
 yeah i'm not sure that's that useful since at that point you should probably be resampling anyway
 @wesm 

any reason u didn't implement this originally?
 Nope
 @snth thanks...fixed up
