Attempt at fixing 'gray' dsl method, and making 'simple-arc.rb' work

Hi,

I have attempted to make one of the samples work - simple-arc.rb:

* Made changes to color.rb to add a lambda to the 'gray' table entry, plus a change to the code to define a method based on the lambda (if present) - may not be the 'best' way to do this in ruby(?)
* Added an 'arc' method to shape.rb.
* Implemented 'arc' in swt/shape.rb (converts radians to degrees, sets 'origin' angle to match reference source).
* Small changes to simple-arc.rb to make it match example image referenced in source file.
* **Possible issue** with change in simple-arc.rb to use @app.width / @app.height as self.width and self.height always seem to be 0 - need to check how this works.

Any feedback gratefully accepted :) (particularly on the changes made to color.rb)

Regards,

Garry.