Fix bug in SparkLoadTask for `CACHE <partitioned_table>`

Each partition would result in a new partitioned table entry ...
Null mergeCombiners will cause NPE in ExternalAppendOnlyMap

in most time , null mergeCombiners works fine if each key is different
from others, but if some key are duplicate , it will cause NPE then the
mergeIfKeyExists is called in ExternalAppendOnlyMap.scala
repair the type mistake from V to Any

repair the type mistake from V to Any
Repair another null mergeCombiner risk

Repair another null mergeCombiner risk
Merge pull request #297 from tianyi/master

Null mergeCombiners will cause NPE in ExternalAppendOnlyMap
FIX: LazyPTFIterator lose the last row when its partitionKey is different from the previous row's
Merge pull request #301 from guowei2/master

FIX: LazyPTFIterator lose the last row when its partitionKey is different from the previous row's
Only set the job description instead of also overriding the job group.
Merge pull request #302 from marmbrus/fixJobGroup

Only set the job description instead of also overriding the job group.
Merge pull request #43 from sundeepn/fifocache

Modified FIFO Cache policy
Allow users to control the number of tasks for table scan operations
This will enable user's queries to perform well even for high cardinality partitioned tables
Merge pull request #304 from sundeepn/fifopolicy

Merge pull request #43 from sundeepn/fifocache
Merge pull request #305 from sundeepn/coalesce

Allow users to control the number of tasks for table scan operations
Merge pull request #296 from harveyfeng/master

Fix bug in SparkLoadTask for `CACHE <partitioned_table>`
Update Spark repo path
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