Patches for partition stats and pruning
Some docs for MemoryTable, updates based on review.
Merge MemoryTableReader and TableReader, plus misc updates based on review.
Merge branch 'master' into part-stats

Conflicts:
	src/main/scala/shark/execution/MemoryStoreSinkOperator.scala
	src/main/scala/shark/execution/OperatorFactory.scala
	src/main/scala/shark/execution/TableReader.scala
	src/main/scala/shark/parse/SharkSemanticAnalyzer.scala
Correctly detect Tachyon connections. Don't run TachyonSQLSuite if Tachyon isn't running...
Merge pull request #235 from harveyfeng/part-stats

Support stats for cached partitioned tables