Add caching information to rdd.toDebugString

I find it useful to see where in an RDD's DAG data is cached, so I figured others might too.

I've added both the caching level, and the actual memory state of the RDD.

Some of this is redundant with the web UI (notably the actual memory state), but (a) that is temporary, and (b) putting it in the DAG tree shows some context that can help a lot.

For example:
```
(4) ShuffledRDD[3] at reduceByKey at <console>:14
 +-(4) MappedRDD[2] at map at <console>:14
    |  MapPartitionsRDD[1] at mapPartitions at <console>:12
    |  ParallelCollectionRDD[0] at parallelize at <console>:12
```
should change to 
```
(4) ShuffledRDD[3] at reduceByKey at <console>:14 [Memory Deserialized 1x Replicated]
 |       CachedPartitions: 4; MemorySize: 50.8 MB; TachyonSize: 0.0 B; DiskSize: 0.0 B
 +-(4) MappedRDD[2] at map at <console>:14 [Memory Deserialized 1x Replicated]
    |  MapPartitionsRDD[1] at mapPartitions at <console>:12 [Memory Deserialized 1x Replicated]
    |      CachedPartitions: 4; MemorySize: 109.1 MB; TachyonSize: 0.0 B; DiskSize: 0.0 B
    |  ParallelCollectionRDD[0] at parallelize at <console>:12 [Memory Deserialized 1x Replicated]
```
I'm not sure what you mean - do you mean "an extremely costly operation"?

Assuming that to be the case, two comments::

 * I though about attaching flags to the function so one could specify the type of debug information desired; I think that makes the function too complex, but I'm hardly firm in that idea.
 * This whole function is specifically to help a developer with debugging.  I don't _think_ having it be costly is all that bad.