Attempt at making simple-curve.rb work

Hi,

I have attempted to make the 'simple-curve.rb' sample work.

At the moment, I've implemented basic 'curve_to' support, and the sample runs giving an image which matches that from the Cairo web-site.

However, I've only managed to have the most 'basic' specs running (Disclaimer:  I'm still relatively new to Ruby), and would appreciate some pointers on which methods / accessors (etc.) should be added.

(any other feedback gratefully accepted :))

Regards,

Garry.