List datasets with CLI

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Requirements

List datasets associated with an organization, user's default organization if none specified explicitly. Provide output in readable tabular format. Provide very high-level details for each dataset. CLI output must fit in 80 char width.

Wireframes

By default list datasets in user's default organization:

$ levar-cli datasets
Datasets for my-org
Dataset                    | Updated    | Type       |    Items
---------------------------|------------|------------|----------
dataset-01                 | 2015-10-10 | classify   |   100200
dataset-02                 | 2015-10-20 | regression |  1002020

Optionally specify organization:

$ levar-cli datasets my-other-org
Datasets for my-other-org
Dataset                    | Updated    | Type       |    Items
---------------------------|------------|------------|----------
dataset-01                 | 2015-10-10 | classify   |   100200
dataset-02                 | 2015-10-20 | regression |  1002020