MAHOUT-1615: drmFromHDFS returning the same Key for all Key,Vec Pairs for Text-Keyed SequenceFiles

SparkContext.sequenceFile(...) will yield the same key per partition for Text-Keyed Sequence files if the key a new copy of the key is not created when mapping to an RDD.  This patch checks for Text Keys and creates a copy of each Key if necessary.    
Yeah- i left it commented for now because there's some work to be done in h2o also.  Introducing the field to the `CheckpointedDrm` trait at math-scala required an implementation in h2o.  So after adding 
```scala
  /** Explicit extraction of key class Tag   */
  def keyClassTag: ClassTag[K] = implicitly[ClassTag[K]]
```
 to `CheckpointedDrmH20.scala`

The tests are failing.  Unfortunately I've not had a lot of uninterupted time to work on this over the last week so I havent really looked at the h2o side yet- not sure yet but i thik we need to do some similar class matching in h2o.