Pytest-mozwebqa

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* Remove credentials support in favour of pytest-variables
* Add support for BrowserStack
* Use pytest-html for HTML reports
* Remove support for Selenium RC

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* Use requests when sending the Sauce Labs result.

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* Support environment variables for Selenium host and port.

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* Only check the base URL from the main process when pytest-xdist is in use.

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* Output request headers when initial test for baseurl fails. Fixes 130

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* Improved handling of Firefox preferences and additional capabilities. Fixes 103 & 104
* Skip destructive tests on firefox.com domain by default
* Use a parameterised test for sensitive URLs
* Support for Selenium EventListeningWebDriver
* Fixed UnicodeDecodeError in HTML report. Fixes 114
* Update the Selenium server used in the Travis-CI tests
* Add simple headless testing support with PhantomJS
* Load Selenium defaults from environment variables
* Adding Appium support for Saucelabs
* Allow overriding the Sauce Labs sharing