add mocha test runner for web

[rodneyrehm]

Currently mocha tests are only run on node. The test.html file is insufficient for proper automated tests. Add a runner to perform tests in a real browser. Extend in-browser-tests to also cover the worker API.

[Zardoz89]

Actually it's possible to run fullt tests with mocha + chai on browser. For example : https://github.com/Zardoz89/filestore-pouchdb/tree/master/test

What I don't know if there is a way to launch Firefox or Chrome on headless mode and get the results of the tests. But at least this, quick manual testing could be donde on the browser.