GDScript Editor Goodies

Some script editor handy features, such as:
- Move Lines: Alt+Arrows ( Move the current line up, down, left or right );
- Clone Line: Ctrl/Cmd+B ( Clone current line );
- Toggle Comment: Ctrl/Cmd+Slash(/) ( Toggle line/selected lines comment on/off );

( I'm re-creating this pull request, the previous one got corrupted and messed up )

![GDScript Editor Goodies](https://cloud.githubusercontent.com/assets/5081992/5012209/fd2d4d7a-6a62-11e4-8542-a7821bf2bffc.png)