CLN: DataFrame move doc from __init__ to cls
This, in part addresses our discussion from #3337.
This PR moves the doc string  from **init** to the more common class location.
(cf.. numpy, rest of pandas classes that have a doc string)
Besides STY / consistency  this would allow for more directly accessing the doc string in a debugging (pdb) session: 

``` Python
print mydf.__doc__
```

Thanks. For consistency's sake, we'll hold off on this until the rest of the major classes 
get the same treatment: Series, Panel, *Index...
 @y-p one approximation to what is to be regarded as 'major classes' might be looking what we find in the pandas name space:

These are the classes for which **doc** is not directly under the class statement, below the rest of the classes.

``` Python
In [11]: [(k, c.__doc__) for k, c in pd.__dict__.items() if k[0].isupper() and c.__doc__ is None]
Out[11]: 
[('WidePanel', None),
 ('SparseTimeSeries', None),
 ('Panel4D', None),
 ('SparseSeries', None),
 ('Timestamp', None),
 ('Period', None),
 ('Series', None),
 ('DateRange', None),
 ('NaT', None),
 ('Panel', None),
 ('TimeSeries', None),
 ('Int64Index', None)]

# here the ones for which __doc__ is under the class statement
In [12]: [k for k, c in pd.__dict__.items() if k[0].isupper() and c.__doc__ is not None]
Out[12]: 
['SparseArray',
 'Categorical',
 'ExcelWriter',
 'DataFrame',
 'Index',
 'SparseList',
 'PeriodIndex',
 'Factor',
 'SparseDataFrame',
 'Term',
 'MultiIndex',
 'HDFStore',
 'DateOffset',
 'SparsePanel',
 'ExcelFile',
 'TimeGrouper',
 'DatetimeIndex']
```

I could do the same for the missing cases listed above. Wdyt?
 Yeah, that'd be good.
That's pretty inconsistent alright.
 The remaining cases are such that constitute pure factory classes employing **new** or have entirely missing doc strings.
Would it make to add empty doc strings in such cases?

``` Python
In [6]: sorted([(c.__module__, k) for k, c in pd.__dict__.items() if k[0].isupper() and c.__doc__ is None])
Out[6]: 
[('pandas.core.daterange', 'DateRange'),
 ('pandas.core.index', 'Int64Index'),
 ('pandas.core.panelnd', 'Panel4D'),
 ('pandas.core.series', 'TimeSeries'),
 ('pandas.sparse.series', 'SparseTimeSeries'),
 ('pandas.tslib', 'NaT'),
 ('pandas.tslib', 'Timestamp')]
```
 Btw. is there a 101 around on how to activate Travis-CI for ones fork?
 http://about.travis-ci.org/docs/user/getting-started/
 `NaT` and `DateRange` don't really need a docstring, the rest do. 
Marking this as 0.11, since there's still a few days until 0.11 final,
and I'll put the missing docstrings together till then.

Thanks.
 Pushed directly to master with some added docstrings.

I believe this is your first commit to pandas. good job! :+1: 
 Thanks @y-p ! ;-)
