Add #inspect for Gradient

Was: <Shoes::Gradient #000000->#ffffff>
Now: (Shoes::Gradient rgba(0, 0, 0, 255)->rgba(255, 255, 255, 255))

I think readability suffers here, but perhaps rgb is actually more
readable. In any case, it's important that the color representations be
consistent, so this matches the current implementation for Color.
Add #inspect for Text, Span, Link

span.to_s    #=> "text"
span.inspect #=> (Shoes::Span:0x01234567 "text")
Add #to_s, #inspect for Point

point.to_s    #=> (40,50)
point.inspect #=> (Shoes::Point:0x01234567 (40,50))