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.
SparkDDLTask(Work) => SharkDDLTask(Work)
Set Shark version to 0.9.0-SNAPSHOT and use Spark's master branch version, 0.9.0-incubating-SNAPSHOT
Merge pull request #183 from harveyfeng/rename-ddl

SparkDDLTask(Work) => SharkDDLTask(Work)
Merge pull request #184 from harveyfeng/patch-1

Set Shark version to 0.9 and use Spark 0.9/master (incubating)
Merge remote-tracking branch 'ampshark/master' into partition-caching

Conflicts:
	src/main/scala/shark/execution/SharkDDLTask.scala
	src/main/scala/shark/parse/SharkDDLSemanticAnalyzer.scala
Finish merging with master branch.
Comments for SharkEnv, SharkConfVars, MemoryStoreSinkOperator.
Comments for RDDUtils, including UnionRDD persistence policy.
Comments for metastore additions
Make CachePolicy optional in PartitionedMemoryTable.
More comments about metastore additions.