Added the Mass Matter component to set the mass and density of a body.

v3/src/physics/matter-js/components/Mass.js

@@ -0,0 +1,21 @@
+var Body = require('../lib/body/Body');
+
+var Mass = {
+
+    setMass: function (value)
+    {
+        Body.setMass(this.body, value);
+
+        return this;
+    },
+
+    setDensity: function (value)
+    {
+        Body.setDensity(this.body, value);
+
+        return this;
+    }
+
+};
+
+module.exports = Mass;