Memory leak

The tool slowly consumes all available memory (24GB) during `fit()`. Version 0.7.
Using as:
```
model = Classifier(
 layers=[
     Layer("Rectifier", units=256),
     Layer("Rectifier", units=256),
     Layer("Rectifier", units=256),
     Layer("Softmax")
 ],
	
 learning_rule='adagrad',
 batch_size=32,
 n_iter=100,
 regularize="L1",
 weight_decay=0.001,
 verbose=True
)
```