Support UICollectionViewDelegate fowarding
[Update] podspec
[Add] Manipulating rows/columns methods.
[Add] Support insert, move, delete rows/columns in HeaderSheet.
[Remove] Insets in Layout.
[Add] Support for BackgroundColor/View in HeaderSheet
[Fix] Change collectionView’s superView from contentView to SpreadSheet itself
[Add] Support for custom UIMenuItem in demo project.
[Add] Support for deleting column in demo project.
0.2.0