Add isDark() and isLight() functions.

spectra.js

@@ -552,6 +552,22 @@
     return this.desaturate(100);
   };
 
+  /**
+   * If a color is dark then it's best to have white text on it.
+   * http://24ways.org/2010/calculating-color-contrast
+   */
+  Spectra.fn.prototype.isDark = function() {
+    var yiq = ((this.red()*299)+(this.green()*587)+(this.blue()*144))/1000;
+    return yiq < 131.5;
+  };
+
+  /**
+   * If a color is light then it's best to have black text on it.
+   */
+  Spectra.fn.prototype.isLight = function() {
+    return !this.isDark();
+  };
+
   /**
    * Returns the color that results from mixing percent of the other color into this color.
    */

test/tester.js

@@ -122,6 +122,9 @@ describe('Spectra', function() {
       expect(color.fadeOut(5).alpha()).toBeCloseTo(0.55, 1);
       expect(color.luma()).toBeCloseTo(77.5, 1);
       expect(color.grayscale().hex()).toBe('#8c8c8c');
+      expect(color.isDark()).toBe(true);
+      expect(color.isLight()).toBe(false);
+      expect(Spectra('#000').isDark()).toBe(true);
     });
 
     it('Mix', function() {