allow Ftp.raw to issue any remote command

add test cases for new raw method
remove superfluous pass definition
preserve error replies on USER/PASS failure

add test coverage for invalid login

fix asylumfunk#4
Merge pull request #81 from asylumfunk/issue/3

Allow sending of arbitrary commands
Merge pull request #82 from asylumfunk/issue/4

Preserve error reply codes during authentication failure.