Add a new DataFrame.dti accessor
There's the `dt` accessor on Series which operates on the Series values:
``` 
pd.Series([1,2,3]).dt.day 
#exists. But checks that series *values*  datetime64[ns]

pd.Series([1,2,3]).day
# doesn't exist
```

There's a top-level method `df.first`:
```
Convenience method for subsetting initial periods of time series data
based on a date offset.
```
which looks like it should live behind `dti` accessor on DataFrame.
```
pd.DataFrame().first(0) 
# method is applicable only for datetime, but is in the top DataFrame namespace
```

There's currently no datetime accessor on dataframe. Would it make sense to move it below a `dti` accessor on DataFrame?  

There's also an unfortunate unfortunate name collision between`Groupby().first`
```
Compute first of group values.
```
and `DataFrame.first`:
```
Convenience method for subsetting initial periods of time series data
based on a date offset.
```
Adding a `dti` accessor to DataFrame would clean that up as well.
