Add unit tests for Graph3D issue

This adds a unit test for PR almende#3255 which fixes almende#3251.
The unit test will fail without the PR merged.

**NOTE:** This also adds module `canvas`, required for the unit test. This module
proved to be quite fickly to install properly. During reviewing, please pay special
attention to the proper installation of this modul. I.e. do following to test:

```
> npm install                      # If no errors, continue
> npm test /tests/Graph3D.test.js  # Run unit test isolated
```

package.json

@@ -57,6 +57,7 @@
     "gulp-util": "^3.0.8",
     "jsdom": "9.9.1",
     "jsdom-global": "^2.1.1",
+    "canvas": "^1.6.5",
     "mocha": "^3.2.0",
     "mocha-jsdom": "^1.1.0",
     "rimraf": "^2.5.4",

test/Graph3d.test.js

@@ -0,0 +1,33 @@
+var assert = require('assert');
+var jsdom_global = require('jsdom-global');
+var vis = require('../dist/vis');
+var Graph3d = vis.Graph3d;
+
+
+describe('Graph3d', function () {
+  var jsdom = jsdom_global("<div id='mynetwork'></div>");
+  var container = document.getElementById('mynetwork');
+  assert(container !== null, 'Container div not found');
+
+  it('accepts new option values on defined instance', function () {
+    var BAR_STYLE = 0;  // from var STYLE in Settings.js
+    var DOT_STYLE = 3;  // idem
+
+    var data = [
+      {x:0, y:0, z: 10},
+      {x:0, y:1, z: 20},
+      {x:1, y:0, z: 30},
+      {x:1, y:1, z: 40},
+    ];
+
+    var options = {
+      style: 'dot'
+    };
+
+    var graph = new vis.Graph3d(container, data, options);
+    assert.equal(graph.style, DOT_STYLE, "Style not set to expected 'dot'");
+
+    graph.setOptions({ style: 'bar'});  // Call fails without PR #3255
+    assert.equal(graph.style, BAR_STYLE, "Style not set to expected 'bar'");
+  });
+});