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).
@pablohoffman, I got it from here: https://github.com/scrapinghub/shubc/blob/master/shubc.go#L52

I guess we should fix shubc too :)