* Implement Bayesian Dropout Cell (credit to Philip Schulz)

* Test for BayesianDropoutCell

* Add --rnn-dropout and --rnn-variational-dropout arguments

* Add varitional dropout on hidden states of RNN

* Added --conv-embed-dropout

* Added embed dropout (source & target). Changed VariationalDropoutCell to ModifierCell. RNNs now always use Variational Dropout BEFORE the residual connection. Added reset method to decoder interface for inference purposes.

* Added RNNDecoder MLP dropout. Refactored context gating and MLP into separate methods. Bumped version.

* adressed Tobis comments