Added publicKey and privateKey settings

As requested in #18, now if you are using SFTP, you can set the path of
your `privateKey` or `publicKey` in the config file - DPLOY will do the
rest.

There’s also one more sweet thing: if you don’t set a password,
publicKey or privateKey, DPLOY will prompt you to type the password
manually. That works for both FTP or SFTP.

Some other methods were refactored and the logs are now improved
(specially to handle errors), so we can debug what happens a bit more.

Resolves #18.
Added support for FTPS

Two new settings for config file were added to support FTPS connections.
Since the library [node-ftp](https://github.com/mscdex/node-ftp) had
support for it, I’ve just added support for those new options.

The `secure` tag is default to `false` and it accepts `true` for both
control and data connection encryption, `control` for control
connection encryption only, or `implicit` for implicitly encrypted
control connection.

The `secureOptions` should be used together with the `secure` mode.