Added a way to change the Assets loader base URL for some special cases

Added a way to change the Assets loader base URL for some special cases overriding the getPreloaderBaseURL() method in the GWTLauncher.
To give the reason, we are working on a 3D view on a website in Ruby/Rails and we cannot put the assets folder at the root, so this change will allow a change on the way to get the assets folder in some advanced integration like this one.
Of course it won't change anything if the method is not overridden.