Support UIViewContentModeCenter with ASImageNode
Merge pull request #144 from facebook/issue-107

Support UIViewContentModeCenter with ASImageNode
Setting a node's background color shouldn't require calling setNeedsDisplay explicitly
Merge pull request #153 from jpasqualini/master

Setting a node's background color shouldn't require calling setNeedsDisplay explicitly
Support editing in table view and collection view
fix unncessary change in editing
Node placeholder API

ASDisplayNodes now have an overidable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asyncronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.