Restore automatic multicore detection for CV

When n.cores is NULL, gbmCluster() detects multiple core automatically
and creates a cluster when appropriate. The code introduced in commit
9519046fe3bb49ad44dc to fix an issue with cluster detection checks
n.cores directly instead of checking the output of gbmCluster(). So
what was automatically parallelised before isn't anymore.

The current code checks the output of gbmCluster and still uses the 
non-parallel lapply() on machines with only one core.
Merge pull request #4 from jiho/fix/automatic_cluster

Restore automatic multicore detection for CV