Libcaca ruby bindings

There is no real documentation yet for the Ruby binding but `methods` on any object should help you :)

I tried to follow Ruby spirit meaning that :

- most of the methods return self
- the methods set_foo with only an argument are also available as foo= (returning the value instead of self)
- the methods originally named get_foo are available only as foo

For the list of methods and some sample code, read:

Libcaca Ruby API