login comand in order to remeber credentials

[oHirokiKumamoto]

This pull request follows #5

Implement new login command:

• It saves credentials to persistent storage
  • For mac: keychain
  • otherwise: encryped json file
• After you finish login command successfully, you don't need to add --user and --password options

Note: The behavior of login command is inspired by heroku login command

[ghost]

Great job! :smile: I've just tried your implementation for a little while and it seems to work, though I'm not so good at node.js as to review your code :-/

[oHirokiKumamoto]

Thank you for feedback. It looks well!