Update API, and add tests for retraining.
Update API, and add tests for retraining.

palashahuja/mlpack,theranger/mlpack,ranjan1990/mlpack,stereomatchingkiss/mlpack,ajjl/mlpack,stereomatchingkiss/mlpack,darcyliu/mlpack,theranger/mlpack,darcyliu/mlpack,ranjan1990/mlpack,darcyliu/mlpack,stereomatchingkiss/mlpack,palashahuja/mlpack,ajjl/mlpack,theranger/mlpack,palashahuja/mlpack,ranjan1990/mlpack,ajjl/mlpack