This patch contains:
**`tests`** - Add unity tests for browserstack and log module.
**`browserstack`** - Add a rest interface request to the browserstack.
**`log`** - Add log decorator for browserstack responses.
**`docs`** - Update docs for pyppium driver and browserstack.
**`driver`** - Update pyppium driver to use environment variables BROWSERSTACK_USERNAME and BROWSERSTACK_ACCESS_KEY