Tail method for spark DataFrame

There is a head method for spark dataframes which work fine but there doesn't seems to be tail method.

```
>>> ans
DataFrame[v1: bigint]
>>> ans.head(3)
[Row(v1=299443), Row(v1=299493), Row(v1=300751)]
>>> ans.tail(3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/jan/git/db-benchmark/spark/py-spark/lib/python3.6/site-packages/py
spark/sql/dataframe.py", line 1300, in _getattr_
"'%s' object has no attribute '%s'" % (self._class.name_, name))
AttributeError: 'DataFrame' object has no attribute 'tail'
```
I would like to feature request Tail method for spark dataframe