ivis: dimensionality reduction in very large datasets using Siamese Networks

New features:

• ivis models are now serializable via pickle/dill/joblib. Thanks to @imatheussm for his contributions toward this.
• The save_model method now accepts an optional "save_format" argument. Setting it to "tfs" will export ivis models in the TensorFlow SavedModel format, which integrates well with other TensorFlow libraries.