This commit cleans up reminders of the node-js test suite removal
by removing the test/node folder.
The test structure has been aligned by moving all tests from
* test/spec/browser to test/spec
* test/spec/integration to test/integration
The project logo now properly embeds in the Viewer and links to bpmn.io.
This way, we are able to ship it as part of a bpmn-js bundle.
In addition to that change this commit upgrades to the latest diagram-js
release, too which adds support for Diagram#destroy().
Closes#15