Objective.name format should be more strict

[varisd]

The Objective.name in general should have more strict naming format, otherwise it cannot be used e.g. for naming graph parts (for example naming Delayed trainer accumulators with corresponding objective names for better debugging)

https://github.com/ufal/neuralmonkey/blob/master/neuralmonkey/trainers/objective.py#L85

At least whitespaces should not be allowed