Issue #2334981 by visuaLatte: Error upon saving webform_ab node if win condition doesn't have options

Issue #2291297 by sheena_d: Added webform_ab_views submodule. Module includes default view, and describes webform_ab database tables to Views.

Added win thresholds: need to be met for all webforms before win conditions are checked. Converted minimum hits to a threshold.

Added hooks to fire when a conversion is recorded, and when test data is reset. Noted these in API.txt (which needs a whole lot of updating).

Added summary for win conditions plugins, to display on admin view of Webform A/B test node. Cleaned up some comments. Always pass ['options'] array to plugin functions

Changed winning_conditions to win_conditions for consistency, and fixed the win condition loading so that the format matches what comes from the node form