Use karma-browserify to run the tests, rather than relying on prebuilt
assets. This commit alters our test harness so that it no longer relies
(at all) on the webassets pipeline. This means:

- test isolation is improved, as each test file runs in its own module
- bringing objects into test scope is now simply a matter of requiring
  the correct module, rather than bolting more and more objects onto the
  Annotator namespace
- autoreloading of tests when either test files or their dependencies
  are modified is now possible (and enabled by default)