Added default title of 'Node'.
Fixed gterm support; quote and join -e argument.
Changed screen(1) names from hN to mininet.hN.
This should make it easier for people who use screen for other purposesto identify which sessions were created by mininet.
Added default prefix for window titles if none is specified.
Fixed regex typo.
Modified to allow execed nodes to work, if we ever reinstate them.
Support gnome terminals too
Caution: code is incomplete - can't set a default type from the CLIand in some places the phrase xterm should be replaced by terminal.