Fix tests (#96)

Closes #63

- Switch from testling to jsdom for browser APIs
- Switch from coverify to nyc for coverage reports
- Clean up related dead code and tooling
- Update Travis to use Node v6 and v8, since we need ES6 features for jsdom.

tests/index.js

@@ -0,0 +1,6 @@
+var jsdomOptions = {
+  url: "https://example.org/",
+  runScripts: "dangerously"
+}
+
+require("jsdom-global")("", jsdomOptions)