[SPARK-2189][SQL] Adds dropTempTable API

This PR adds an API for unregistering temporary tables. If a temporary table has been cached before, it's unpersisted as well.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #3039 from liancheng/unregister-temp-table and squashes the following commits:

54ae99f [Cheng Lian] Fixes Scala styling issue
1948c14 [Cheng Lian] Removes the unpersist argument
aca41d3 [Cheng Lian] Ensures thread safety
7d4fb2b [Cheng Lian] Adds unregisterTempTable API