Add test/support directory, shared equalHTML assertion

Update the Brocfile to build a test/support directory as ES6 modules,
making them available to tests.
Wrap appendChild in dom helper
Move cloneNode into dom helpers
Merge pull request #35 from mixonic/test-support

Add `test/support` directory, shared `equalHTML` assertion
Merge pull request #36 from mixonic/dom-helpers

Wrap dom API access in helpers