Add login command

One step at a time. :)

This PR adds a `login` command that stores the SH's key in the `netrc` file.

Also, I've improved the management and dependency control of commands so we don't need to repeat common code.

Finally, a function to read the key from several sources was added. This functions should be useful in other commands (I'm currently using it on the `log` command).
Right, I've forgotten to add this feature.

Regarding the auth issue, I think this is a good idea. `shub` and `scrapy` are different projects, so it makes sense for me not sharing configuration files between them. Also, in the future we might want the user to execute `shub login` to configure more options (other than SH's key) before start using other commands.