Print Screen - output to terminal

I run my pw manager on a colo'd box with no X.  I understand the threat model and I want to output my passwords to the terminals anyways.  I added the "print_screen" function (alias "ps") to print out a password.  After the given number of seconds, if an interactive terminal is detected, the password is overwritten using shell escapes (the same way a password line is reused in the ask_for_password method earlier).

Again, missing tests - if you want to accept this, let me know, and I'll write some tests and a cleaner diff for you.

Thanks!
-Carl