Can now specify whether the height of the views should be set as equal

I've been using PureLayout in a project recently (a major time saver - thanks) and a couple of times came across some challenges where the default behaviour for `autoDistributeViewsAlongAxis:withFixedSpacing:insetSpacing:alignment:` is trying to make each view the same height. That is often, but not always, the desired behaviour.

On my local fork, I've added the ability to pass a boolean to indicate whether you want the views to be constrained to be the same height.  If this is useful, feel free to pull in... otherwise, just ignore.

Cheers,
Craig