Allow to set just the gradient colors, without locations, relying on CAGradientLayer's
uniform color distribution logic.
Also update all the uses of GradientView to only specify colors because all the
places were using just two locations: 0 and 1.
GradientView would not work if its colors were passed during initialization
and never changed after. This was caused by the fact that `CAGradientLayer`'s
colors were updated in `GradientView.colors.didSet`, which isn't
called during `init()`.
Also allow access to `GradientView.gradientLayer` so that users of
that class can build more advanced gradients.
