Double click support on selected item
Refactoring in MMCoverFlowLayer and MMFlowView

- new datasource method in MMCoverFlowLayerDataSource: -coverFlowLayer:willShowLayer:atIndex:, will be invoked for every visible layer on relayout
- refactored image updating in MMFlowView to use new method, removed -updateImages
- better and cleaner tests
- removed previewScale property from MMFlowView
- deleted unneeded code
Refactored MMFlowViews MMCoverFlowLayerDatasource methods into category

- cleaned up tests