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?

we'd want to update the deploy command to use this function.

also, we'll want to support a configuration file for `shub` cli tool, the support of reading api keys from `scrapy.cfg` in deploy command should be meant only for backward compatibility purposes.