tcp_socket option should not be required in attach command

[mit-mit]

Currently the command for attaching to a device is: fletch attach tcp_socket <ip address>

As attaching over tcp is likely the most common way of attaching, it would be very nice if the command could be simplified to: fletch attach <ip address>

[kasperl]

Also, we could consider shortening tcp_socket to tcp -- I'm not sure there's any other tcp-related thing we could attach to.