Deepreplay

This small release includes support for custom metrics and objects.
Prior to this release, DeepReplay expected a metric name like `acc` as argument and would fail if an instance of a metric object were passed as argument instead.
Now, it takes either a metric name or an instance of a metric object (custom or not).

Fixes
- Setting default movie writer back to FFMPEG
- Applied identity function to input tensor to comply with changes introduced by TensorFlow 1.8 - thanks to albb762 :-)

ReplayData Callback
- `model` parameter included, for fetching model initial conditions without having to fit it

Replay
- new visualizations included:
- violin plot for the weights (`build_weights` method)
- violint plots for the Z-values/Activations (`build_outputs` method)
- violin plot for the gradients (`build_gradients` method)
- new properties
- `weights_std`: standard deviation of the weights per layer
- `gradients_std`: standard deviation of the gradients per layer

Datasets
- two new datasets included:
- `ball`for generating points with zero mean and unit standard deviation inside a n-ball
- `hypercube` for generating points for the corners of a n-dimensional hypercube