Add a SparkDDLTask for Shark-specific metastore operations.
Includes support for ALTER TABLE <old naem> RENAME TO <new name>.

Currently, this is meant to be a dependent task on Hive's DDLTask.
Unit test for ALTER TABLE RENAME TO on cached tables.
Fix merge mistake in SharkDDLSemanticAnalyzer.
Make SparkDDLTask#alterTable() more general.

+ Only create a SparkDDLTask for cached table ops.
Merge pull request #181 from harveyfeng/spark-ddl-task

Add a SparkDDLTask for handling Shark-specific metastore operations.