[SPARK-5588] [SQL] support select/filter by SQL expression

```
df.selectExpr('a + 1', 'abs(age)')
df.filter('age > 3')
df[ df.age > 3 ]
df[ ['age', 'name'] ]
```

Author: Davies Liu <davies@databricks.com>

Closes #4359 from davies/select_expr and squashes the following commits:

d99856b [Davies Liu] support select/filter by SQL expression