Ivis

This release introduces a number of new features into `ivis`:

* Windows support
* Code changes to support `ivis` on Python2
* R package received a major facelift - with big thanks to JOSS reviewers
* Added cosine distance metric in triplet loss function
* Minor bug fixes and performance improvements

Appropriate contribution assignments.

1.2.3-joss
`ivis` release following feedback from JOSS review.

Support for sparse matrices in supervised mode
Bug fixes

Added callbacks and sanity checks during module imports

Bug fixes and cleanup

Supervised mode added to ivis. Additional features:

* Add ``classification_weight`` parameter to allow users to tune balance between classification vs. triplet loss.
* Add Ivis callbacks module for ivis-specific callbacks such as checkpointing during training. Ivis object code changed to deal with provided callbacks.
* Tensorboard callbacks
* Sparse matrix support in supervised mode