Support leading/trailing edges

This is a great UIView category. It seemed like it would be simple to add support for leading/trailing edge constraints. I've copied Demo4 and made a Demo6 that uses leading instead of left and trailing instead of right. I also added an Arabic localization to test it out, with different edge insets for leading and trailing edges. In addition I switched on the status bar in the example project so I can check the effect of the expanding status bar when the phone gets a call. Left and right edges still work, but leading/trailing now provide automatic support for localisation to non-English locales.