Needle

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- Dropped Python 2.6 support.
- Verified support for Python 3.2-3.6.
- Firefox improvements: use FirefoxWebElement and geckodriver by default.

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- Fixed a race condition in creating baseline and output directories.

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- Added support for Selenium 3.
- Added ImageMagick engine.
- Fixed an issue with false positives yielded by the Perceptual engine.

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- Added the cleanup_on_success option for deleting screenshots after successful test runs.

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- Use selenium native element location instead of javascript call in order to assure crossplatform compatability.

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- Fixed a unicode error (issue 23).