Handling int-response during classification.

Now during classification int-vector is transformed into
proper "enum":
 - int domain is collected (e.g., [1,2,3,5]), transformed to enum domain
(["1","2","3","5"]) and the vector is wrapped into TransfVec which
makes adaptation (1->0, 2->1, 3->2, 5->3).

- the same process is applied to response in validation dataset

- proper validation response alignmnet respecting model response is
  performed

- get a rid of _ymin property in TreeModel

- WARNING: there is a key leak

Conflicts:
	src/main/java/water/util/Utils.java