Added NeptuneSaver for logging model checkpoints

[jakubczakon]

Fixes #

• Added NeptuneSaver for logging model checkpoints to neptune.
• In all examples, I've added token for anonymous user neptuner to makes things easier for new-comers to see how it works.

Description:
NeptuneSaver can be used like DiskSaver

to_save = {'model': model}
handler = Checkpoint(to_save, NeptuneSaver(npt_logger), n_saved=2,
                     filename_prefix='best', score_function=score_function,
                     score_name="validation_accuracy",
                     global_step_transform=global_step_from_engine(trainer))
validation_evaluator.add_event_handler(Events.COMPLETED, handler)

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

@jakubczakon thanks for the PR, I let you fix failing tests and we are good, I think

[jakubczakon]

@vfdev-5 thanks!
I may change/fix one more thing on neptune side.

[jakubczakon]

@vfdev-5 ok, so I fixed tests and added the change I wanted in-line with the update on the neptune-client side.

[vfdev-5]

Looks good ! Thanks Jakub!