Move terminal.gd and pty.gd to godot_xterm directory

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.

examples/socat_terminal/socat_terminal.gd

@@ -1,4 +1,4 @@
-extends "res://addons/godot_xterm/nodes/terminal/terminal.gd"
+extends "res://addons/godot_xterm/terminal.gd"
 
 export (String) var exec_path := "bash"
 export (String) var socat_path := "socat"  # E.g. /usr/bin/socat

examples/socat_terminal/socat_terminal.tscn

@@ -10,5 +10,4 @@ script = ExtResource( 1 )
 __meta__ = {
 "_edit_use_anchors_": false
 }
-
 [connection signal="data_sent" from="." to="." method="_on_Terminal_data_sent"]