Initial attempt to make the 'simple-curve' sample work as an introduction
to Shoes.
Have added a 'Curve' class to lib/shoes and lib/shoes/swt.
This seemed to be the 'simplest' implementation to display an image
matching that on the cairographics samples.

Possible immediate issues:
* Should use Point instead of x1, y1 etc.
* Doesn't calculate the bounds
Updated curve to use Point rather than individual X,Y coords
Renamed parameters to Curve.initialize (now uses point_1, control_1, control_2, point_2)
Added 'curve' to dsl.rb
Improved the 'curve' Mock.
Updated swt/curve.rb to implement a basic painter, set up a 'path' and extract the 'bounds' from the 'path'.  Added attr_reader for left, top, width, height.
First step of cleanup of useless code.
Amended curve_to to update the extent of the shape.
Renamed update_bounds to update_bounds_rect
Added new update_bounds as a utility method to work in terms of arrays of X and Y coords (essentially the body of the 'old' update_bounds method).  Now called from update_bounds_rect and curve_to.
Merge pull request #295 from GazCMarsh/master

Attempt at making simple-curve.rb work