update README to reflect new 'raw' syntax

This PR updates the README documentation concerning the extension made to the `Ftp.raw` command at b64377d84afac7e7aec29db39476b3fb31ddc373.

This (`Ftp.raw("command", params, callback)`) is the preferred syntax as it supports all possible server commands, whereas `Ftp.raw["command"](params, callback)` only supports a subset of the specification.  Calls to the former method will always return the corresponding server response, whereas calls to the latter will throw a `TypeError` for non-existing methods. As mentioned in sergi#81, the latter method is still supported, though it should be considered deprecated.