Makes for pretty paths when extending scripts:
`extends "res://addons/godot_xterm/terminal.gd"`
vs.
`extends "res://addons/godot_xterm/nodes/terminal/terminal.gd"`
Currently "res://addons/godot_xterm/pty.gd" is acutally `pty_unix.gd`.
This is okay for now as the PTY node is only supported on Unix
platforms. However, we will need to sort it out when adding Windows
support as part of #25.
Also remove the GDXterm namespace.
Uses OS.execute to start socat with a tcp server on one end and shell on
te other.
Connects to the socat server using StreamPeerTCP.
Doesn't support reszing and inherits 'dumb' for the TERM env var so
doesn't work too well without updating TERM and resizing with stty.
