[SPARK-2227] Support dfs command in SQL.

Note that nothing gets printed to the console because we don't properly maintain session state right now.

I will have a followup PR that fixes it.

Author: Reynold Xin <rxin@apache.org>

Closes #1167 from rxin/commands and squashes the following commits:

56f04f8 [Reynold Xin] [SPARK-2227] Support dfs command in SQL.

(cherry picked from commit 51c8168377a89d20d0b2d7b9a28af58593a0fe0c)
Signed-off-by: Reynold Xin <rxin@apache.org>