Fix some issues with the addition of leading and trailing

- Fix issues with the absolute positioning method (was not actually respecting the edge passed in to it) and add support for leading/trailing edge
- Add support for right-to-left languages when using the method to distribute subviews with fixed size
- Add support for leading and trailing when aligning views to each other
- Fix up some comments
- Showing the status bar on iOS 7 requires that the content receive an additional top inset so it does not appear underneath the status bar
Update README now that the API supports Leading and Trailing attributes