fixed: passing null parameter to HashSet when traininig sample for a user is empty
fixed: random instance need to be initialized to perform incremental modeling(AddFeedback) in BPRMF
fixed: To prevent passing null parameter to HashSet<int> for user who no samples in training data
fixed: Without Train(), BPRMF's random instance is not instantiated.

To perform incremental modeling on the loaded model(LoadModel), random
instance need to be instantiated on LoadModel().
Merge branch 'master' of https://github.com/ummae/MyMediaLite
Merge pull request #427 from ummae/master

Evaluation of unseen user and Incremental modeling issues in BPRMF