Add password prompt feature to FTP deployment providers

This commit removes the requirement to store FTP passwords in beam.json
files. A password is now prompted for where it is not specified in the config.

Resolves #38
Fix display of a stack trace when running in verbose mode

Default Symfony Console 2.2 behaviour is to show a full stack trace when
running in verbose mode. This was being overridden by a basic error message
inside TransferCommand.