added example to demonstrate and test gradient mixin

examples/css3/config.rb

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+# Require any additional compass plugins here.
+project_type = :stand_alone
+css_dir = "stylesheets"
+sass_dir = "src"
+images_dir = "images"
+relative_assets = true

examples/css3/src/gradient.sass

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+@import compass/css3/gradient.sass
+
+div
+  width: 200px
+  height: 100px
+  border: 1px solid #777
+.linear
+  +v-gradient(#fff, #aaa, color_stop(50%, #ccc, 50%, #bbb))
+.radial
+  +radial-gradient("45 45, 10, 52 50, 30", Cyan, DodgerBlue)