ENH: allow construction of datetimes from columns in a DataFrame
closes #8158

See the references SO questions in the issue. but allows highly performant construction of datetime series from specified DataFrame columns with a minimal syntax

```
In [12]: pd.options.display.max_rows=10

In [13]: year = np.arange(2010, 2020)

In [14]: months = np.arange(1, 13)

In [15]: days = np.arange(1, 29)

In [16]: y, m, d = map(np.ravel, np.broadcast_arrays(*np.ix_(year, months, days)))

In [17]: df = DataFrame({'year' : y, 'month' : m, 'day' : d})

In [18]: df
Out[18]: 
      day  month  year
0       1      1  2010
1       2      1  2010
2       3      1  2010
3       4      1  2010
4       5      1  2010
...   ...    ...   ...
3355   24     12  2019
3356   25     12  2019
3357   26     12  2019
3358   27     12  2019
3359   28     12  2019

[3360 rows x 3 columns]

In [19]: pd.to_datetime(df, unit={ c:c for c in df.columns })
Out[19]: 
0      2010-01-01
1      2010-01-02
2      2010-01-03
3      2010-01-04
4      2010-01-05
          ...    
3355   2019-12-24
3356   2019-12-25
3357   2019-12-26
3358   2019-12-27
3359   2019-12-28
dtype: datetime64[ns]

In [20]: %timeit pd.to_datetime(df, unit={ c:c for c in df.columns })
100 loops, best of 3: 2.33 ms per loop

# we are passing a dict of mapping from the df columns to their units.
# obviously also includes hours, min, seconds, ms, etc. as well as aliases for
# these (e.g. H for 'hours'). I wrote them out to avoid confusion of ``M``, is this Month or Minute. 
# could also accept ``%Y`` for the strptime mappings.
In [21]: { c:c for c in df.columns }
Out[21]: {'day': 'day', 'month': 'month', 'year': 'year'}

```

cc @unutbu 
cc @jakevdp 

@shoyer @jorisvandenbossche 
 Rather than the `pd.to_datetime(df, unit={'year': 'year', 'month': 'month', 'day': 'day'})`, why not spell this `pd.to_datetime({'year': df.year, 'month': df.month, 'day': df.day})` (`pd.to_datetime(df[['year', 'month', 'day']])` also works)?
 +1 on @shoyer's suggestion. Seems very clean and intuitive for the common case of converting columns to a date.
 @shoyer ok that is a nice spelling. Only concern is the orderings of a DataFrame columns when they are positionally based

IOW,

`pd.to_datetime(df[['year', 'month', 'day', 'hour', 'minute', 'second', 'millisecond', 'microsecond', 'nanosecond']])`, but could spell that out in the doc-string.

I suppose for the common case this makes sense.
 Why does the order matter? I haven't looked at your implementation yet, but my approach would be to simply try to pull columns out of the DataFrame/dict by name (e.g., `hour = arg.get('hour', 0)`)
 @shoyer ahh good point, but that requires they be named exactly right. Ok can just treat it like a dict then. Last issue is what if MORE things (e.g. unrecognized are passed), e.g. 

`{'year' : ..., 'month': ..., 'day' : ..., 'the_hour' : ...}`. I _think_ this should raise as this is a user error.
 > Last issue is what if MORE things (e.g. unrecognized are passed)

I think an error is appropriate in this case. It would force users to be explicit about which columns they want used, which is good.
 ```
In [1]:         df = DataFrame({'year': [2015, 2016],
                        'month': [2, 3],
                        'day': [4, 5],
                        'hour': [6, 7],
                        'minute': [58, 59],
                        'second': [10, 11],
                        'ms': [1, 1],
                        'us': [2, 2],
                        'ns': [3, 3]})

In [2]: df
Out[2]: 
   day  hour  minute  month  ms  ns  second  us  year
0    4     6      58      2   1   3      10   2  2015
1    5     7      59      3   1   3      11   2  2016

# I know a typo....
In [3]: pd.to_datetime(df.assign(foo=1))
ValueError: extra columns have been passedto the datetime assemblage: [foo]

In [4]: pd.to_datetime(df[['month','year','day']])
Out[4]: 
0   2015-02-04
1   2016-03-05
dtype: datetime64[ns]

In [5]: pd.to_datetime(df)
Out[5]: 
0   2015-02-04 06:58:10.001002003
1   2016-03-05 07:59:11.001002003
dtype: datetime64[ns]
```
 @jreback haven't looked yet, but what happens on `pd.to_datetime(df[['year', 'month', 'day']])` when one (or more) of those columns have duplicates? We should catch that early and raise.
 @TomAugspurger pushed a nicer error msg if dups are passed.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/12967?src=pr) is **83.86%**

> Merging [#12967](https://codecov.io/gh/pydata/pandas/pull/12967?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #12967   diff @@
==========================================
  Files           136        136          
  Lines         49711      49750    +39   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          41679      41720    +41   
+ Misses         8032       8030     -2   
  Partials          0          0          
```
1. File `...das/tseries/tools.py` was modified. [more](https://codecov.io/gh/pydata/pandas/commit/521b8af92cf3125bc2a9cb4f2e0e1e3dfa4951d3/changes?src=pr#70616E6461732F747365726965732F746F6F6C732E7079) 
   - Misses `-4` 
   - Partials `0` 
   - Hits `+4`

[![Sunburst](https://codecov.io/gh/pydata/pandas/pull/12967/graphs/sunburst.svg?size=660&src=pr)](https://codecov.io/gh/pydata/pandas/pull/12967?src=pr)

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by 521b8af
 ok, the only thing missing is a nice way to tell the user acceptable units (maybe in the docs). I trivially did it in the doc-string (Example), but should we add a list somewhere?
 Some further comments after merge: +1, this is a nice enhancement!

But, my main comment is that I would restrict the possibilities. I would just let users have to provide very explicit dict keys/column names (only year, month, day, hour, minute, ... + maybe the plural forms). This makes the method less magical (and solves the documentation issue you noted), and also avoids possible confusing about `m` not allowed/being month or minute. 
 @jorisvandenbossche ok, let me see with a followup for that.
