Tree model contains keys instead of compressed models.

PUB-308: the problem was that a tree model became to large
to be stored in DKV because of generated huge trees.

Hence, the fix saves each individual tree as
an individual object into DKV and model holds only a list of trees' keys.
On the other hand, a tree model has to make DKV#get to obtain a tree, hence it preserve
a tree cache for already loaded trees.
Downgraded sbteclipse version since the newest does not handle project
dependency in correct way.
Proper clean-up of tree-keys + handling binary classifiers properly.

PUB-308
 - delete of model should invoke delete of all trees which are hold by
   the model
 - avoiding NPE in lazy tree load since binary classifiers do not
store the second tree explicitly.
Merge branch 'master' of github.com:0xdata/h2o
Merge branch 'master' of github.com:0xdata/h2o