Merge pull request #220 from harveyfeng/rewrite

AST transformations for DISTINCT aggregates
Conflicts:
	src/main/scala/shark/SharkDriver.scala
Merge pull request #267 from sundeepn/astrewritefix

AstRewrite for count distinct can cause unexpected issues with certain queries
Patch 'null' prompts for non "shark" exec modes
Disable rewrites for rollup, cubes, and limits.

We also output a warning when a LIMIT is used for COUNTs
Merge pull request #300 from harveyfeng/rewrite-09

AST transformations for DISTINCT aggregates