[SPARK-6957] [SPARK-6958] [SQL] improve API compatibility to pandas

```
select(['cola', 'colb'])

groupby(['colA', 'colB'])
groupby([df.colA, df.colB])

df.sort('A', ascending=True)
df.sort(['A', 'B'], ascending=True)
df.sort(['A', 'B'], ascending=[1, 0])
```

cc rxin

Author: Davies Liu <davies@databricks.com>

Closes #5544 from davies/compatibility and squashes the following commits:

4944058 [Davies Liu] add docstrings
adb2816 [Davies Liu] Merge branch 'master' of github.com:apache/spark into compatibility
bcbbcab [Davies Liu] support ascending as list
8dabdf0 [Davies Liu] improve API compatibility to pandas