Support for in-memory, Hive-partitioned tables.

Note: dependent on https://github.com/amplab/shark/pull/181

Added along the way:
- Creating tables without CTAS - e.g., CREATE TABLE test_cached (...) [PARTITIONED BY (...)]
- General cache policy interface and LRU implementation, since there isn't a good way to specify, from the CLI, which partitions to keep in memory.
 Table class, with MemoryTable and PartitionedMemoryTable subclasses, for containing RDD metadata.

Partition pruning and stats collection is still unsupported/untested. Also, some additions made a few files, especially MemoryStoreSinkOperator, more confusing. Planning to submit fixes for these two issues in another CL, since it might involve more refactoring.