Various improvements + Adding shortcuts for labels & images

Hey Charles,

thanks for your Linear Layout View! I built something similar some time ago. But you approach (with wrapped LayoutItems) is better than mine. Still I had some Ideas, that would also improve your Layout class. I would love, if you would merge them.

There are 6 commits, the commit message explains most of them. But here are some more comments:

e39bf56 fixed missing scroll indicators:
The ScrollIndicators are actual subviews of a UIScrollView, so you remove them in removeAllItems currently.

ee2482a respect scrollView content insets:
The sizes are currently wrong, if so. uses insets on the scrollView

cda57ed performance increase | only relayout, if needed:
All of your layout code gets calles on every scrolling movement (change in contentOffset). This results in a quite bad scrolling performance (depending on device and count of layoutItems)


98ac5a2 added adding shortcuts for text & images:
I like to have a fast way to display some images and texts in a UIScrollView - if you don't think this should be part of your class, leave this commit out. But I like it very much.

Cheers Markus (@jaydee3 on twitter)