Flow layout contentSize computation improvements.

- fixes premature cell removal when alwaysBounce* is enabled #117
fix unused variable warning

ifdef the whole loop in NS_BLOCK_ASSERTIONS
to prevent an unused variable warning.
Merge pull request #118 from diederich/develop

fix unused variable warning
Fix some storyboard incompatibility and collection view behaviours.
Merge pull request #128 from beingenious/master

Fix some storyboard incompatibility and collection view behaviours.
implemented scroll to collection view scroll positions for scrollToItemAtIndexPath:atScrollPosition:animated:
change orientation check to UIInterfaceOrientationIsLandscape()
fix convertRect to use bounds instead
better comment
fixes some typos and casts
remove codes! new calculation independent of frame.origin.x/y
merged @steipete changes for bitwise check, vertical/horizontal cases, spacing. removed addition of interim/line spacing to match uicollectionview behaviour. merged to the new calculation without using bounds origin.

merged @steipete changes for bitwise check, vertical/horizontal cases,
spacing. removed addition of interim/line spacing to match
uicollectionview behaviour. merged to the new calculation without using
bounds origin.
Merge pull request #129 from alvinsj/add-scrollposition

implemented scroll positions for scrollToItemAtIndexPath:atScrollPosition:animated:
Update README.md
Justify one item row to center
Modify frame/center instead of layer.frame/layer.position.
Changing the CALayer's frame instead of the UIView's frame prevents the subviews from resizing correctly. This can result in an incorrectly-sized contentView and other views that have been added to the view hierarchy.
Merge pull request #140 from ksuther/master

Modify UIView's frame instead of CALayer's frame in applyLayoutAttributes:
Fix linelayout example by removing invalid bound checks
Merge pull request #141 from taiyangc/fix_linelayout

Fix linelayout example by removing invalid bound checks
_visibleBounds changed to self.visibleBoundsRect