Combined changes: refactored helpers, support alternative tools

[acdha]

This is my working branch with various changes:

• Refactored Selenium setup into a setUpClass method, which dramatically improves run-time since the Selenium session overhead is massive
• Allow baseline-capture to be requested by NEEDLE_CAPTURE env. variable
• Select a browser other than Firefox using the NEEDLE_BROWSER env. variable
• Store screenshots in screenshots/ subdirectory; allow overriding by setting the NEEDLE_OUTPUT_DIR and NEEDLE_BASELINE_DIR env. variables
• Automatically create output/baseline directories if they do not exist
• Refactor assertScreenshot core into compareScreenshot context manager which allows the check for a baseline image to be done before actually loading the page in a browser which is much faster when only a small percentage of the baseline images need to be re-captured
• Set NEEDLE_USE_PERCEPTUALDIFF to use http://pdiff.sf.net instead of the built-in comparison
• Force the Selenium browser viewport to be a specific size (defined as the TestCase viewport_height and viewport_width class properties) which makes cross-browser/platform image comparison more realistic as e.g. scrollbar dimensions won't cause all of the images to be different width.

[bfirsh]

Brilliant, thank you! Needs some docs, but we can add that later.