SPARK-1999: StorageLevel in storage tab and RDD Storage Info never changes

StorageLevel in 'storage tab' and 'RDD Storage Info' never changes even if you call rdd.unpersist() and then you give the rdd another different storage level.

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #968 from CrazyJvm/ui-storagelevel and squashes the following commits:

62555fa [CrazyJvm] change RDDInfo constructor param 'storageLevel' to var, so there's need to add another variable _storageLevel。
9f1571e [CrazyJvm] JIRA https://issues.apache.org/jira/browse/SPARK-1999 UI : StorageLevel in storage tab and RDD Storage Info never changes
SPARK-2148 Add link to requirements for custom equals() and hashcode() methods

https://issues.apache.org/jira/browse/SPARK-2148

Author: Andrew Ash <andrew@andrewash.com>

Closes #1092 from ash211/SPARK-2148 and squashes the following commits:

93513df [Andrew Ash] SPARK-2148 Add link to requirements for custom equals() and hashcode() methods
Updating docs to include missing information about reducers and clarify ...

...how the OFFHEAP storage level works (there has been confusion around this).

Author: Ali Ghodsi <alig@cs.berkeley.edu>

Closes #1089 from alig/master and squashes the following commits:

ca8114d [Ali Ghodsi] Updating docs to include missing information about reducers and clarify how the OFFHEAP storage level works (there has been confusion around this).
SPARK-2039: apply output dir existence checking for all output formats

https://issues.apache.org/jira/browse/SPARK-2039

apply output dir existence checking for all output formats

Author: CodingCat <zhunansjtu@gmail.com>

Closes #1088 from CodingCat/SPARK-2039 and squashes the following commits:

c52747a [CodingCat] apply output dir existence checking for all output formats