Firefox browser uses download profile by default

A few other browser tweaks:
- Moved atexit (and related quit function) from fixtures.browser to utils.browser
- Added docstrings!
- ensure_browser_open now returns the browser is ensures is open, so you don't also have to import browser to use it
- The ability to pass a webdriver class to start() has been removed in favor of centralizing configuration entirely in env.yaml
Merge pull request #367 from seandst/firefox-profile

Firefox browser uses @mfalesni's download profile by default
Browser configuration docs
Merge pull request #368 from seandst/browser-configuration-docs

Browser configuration docs