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
Updated tabstrip with new element() parameters
Merge pull request #376 from mfalesni/tabstrip_update

Updated tabstrip with new element() parameters
Fix for cloud provider tests
Merge pull request #378 from psav/fix_cloud_prov_tests

Fix for cloud provider tests
Fixed a cloud prov error
Merge pull request #379 from psav/fix_cloud_prov_tests

Fixed a cloud prov error
Fix empty credential on cloud provider discovery
Merge pull request #380 from weissjeffm/master

Fix empty credential on cloud provider discovery
Merge pull request #367 from seandst/firefox-profile

Firefox browser uses @mfalesni's download profile by default