Einops

- experimental support for Jax framework was added
- testing code was rewritten and updated to work
- tf2 always worked with einops, but tests had to be updated. So, tests are updated for tf2
- tf readme, minor additions, comments, etc.

Thanks to [contributors](https://github.com/arogozhnikov/einops/graphs/contributors)

This release introduces einops, as well as its notion.

Initial release API:
Operations (ops)
- `einops.rearrange` and `einops.reduce`

Auxiliary

- `einops.asnumpy` and `einops.parse_shape`

Layers (for chainer, gluon, keras and torch)

- `Rearrange` and `Reduce`

Supported frameworks:

- numpy
- pytorch
- tensorflow eager
- cupy
- chainer
- gluon
- tensorflow
- mxnet (experimental)
- and keras (experimental)