Add some misc. comments in TableScanOperator.
Merge branch 'master' of git://github.com/amplab/shark

Conflicts:
	src/main/scala/shark/execution/TableScanOperator.scala
Comments for MemoryMetadataManager::unpersist()
Update some misc comments for TableScanOperator/MetadataManager
Merge pull request #145 from harveyfeng/partition-caching

Some misc. comments in TableScanOperator and MemoryMetaDataManager