DNM: Optional password

We remove the need to save the password explicitly in the configuration file. Firstly, when user initialises a project, she is asked whether her password is to be saved to the configuration file. If not and she invokes the tx command, then she is asked for the password the first time it is needed. She won't be asked again until the command's invocation finishes. Of course if the password is already in configuration file, the user can manually remove it with the same effects.

@diegobz Please review this