Add math/sript expression support in property editor

![](http://i59.tinypic.com/x5p94l.jpg)
Simple math expression

![](http://i58.tinypic.com/1zegn14.jpg)
Complex math expression

![](http://i57.tinypic.com/2lwavk7.jpg)
Reference editing node with "this" 

![](http://i58.tinypic.com/1ovafn.jpg)
Reference any property of any node

![](http://i59.tinypic.com/wk5ous.jpg)
Or script variable

- Since the implementation is using Godot's script interface, any valid script expression could be used
- Cons:
  Script been re-created for each eval operation;
  "this" keyword might be confusing