added a color cache for rgba colors
Improved Color.make_sdl_rgba caching a bit. (Simpler code; freezes result)
Added #hash methods for all color classes.
Uncache color names when their value changes.
All methods that take a filename convert it to string.

For consistency with 2.5, and to allow using Pathname, etc.
Added "ri" rake task to generate RI-formatted docs.
Added new argument style for Surface.new.

It can now take an options hash instead of depth and flags.
Giving depth and flags is deprecated now.
Added new argument style for Screen.new.

Same new style as Surface.new.
Updated GL demos to new Screen syntax.
Merge branch 'maint-2.6' into next
TTF#initialize calls TTF.setup automatically. [#32]
Added Surface#bezier_curve.
Renamed Surface#bezier_curve to #draw_curve and improved it.

 - Clarified "weird_parameter" (it's "steps", or quality).
 - Set default quality to 5.
 - Set minimum quality 2. (Smaller values are invisible.)
 - Improved docs somewhat.
 - Returns self, like the other draw methods.
Added Screen.icon=, class method version of Screen#icon=. [#7]
Surface #dup and #clone now make a real (deep) copy.
Added #dup and #clone stub methods for documentation purposes.
Screen#convert returns a Surface instance.
Screen#dup and #clone return Surface instances.
Surfaces now support Marshal.dump and Marshal.load. [#1]
Screen class cannot be marshalled or unmarshalled.

It would just be asking for trouble.