add 'log' command to get the log records of a given Scrapy Cloud job

@pablohoffman, do you intend to support common operations in Scrapy Cloud, like scheduling and getting the log records of a given job? If your answer is yes, here is a PR that uses the [scrapinghub] (https://github.com/scrapinghub/python-scrapinghub) library do get the log records of a given job.

Please, take a look at the line 2 of shub/log.py (copied from deploy.py). Is __setuptools__ needed in all commands or it's a just a __deploy__ specific requirement?

If we're gonna be outputting logs in plain text format, I think it's better to retrieve the log directly in text format from hubstorage api. This avoids: unnecessary conversion from json, unnecessary line formatting and inventing a new text format.

To retrieve logs in text you can either pass `format=text` to storage API or `text/plain` in `Accept-Encoding` header.