More configurations and bug fix

• Minimum content height configuration (#84)
• Minimum column spacing per section configuration (#86)
• Fix a crash when no data and no section (#76, #93)
• Swift update (#77, #92, #98)
• Long cell disappearing fix (#78)
• Fix height calculation (#79)
• Pixel perfect (#83)

More configurations and Swift version

• Adding a configuration option to determine the order of rendering the items. Options include Left-to-Right | Right-to-Left | Shortest column first (existing default).
• New demo screenshots
• Custom column count per section
• Swift version
• Code cleanup

More delegate methods and Travis-CI

• Stability improvement
• Integrate Travis-CI
• Add more delegate methods

Bug fix and API change

• [Add] Add minimumColumnSpacing and minimumInteritemSpacing properties.
• [Remove] Remove itemWidth property. The layout object will calculate a proper item width automatically.
• [Change] Rename delegate method - (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout heightForItemAtIndexPath:(NSIndexPath *)indexPath to - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath. It should return original size for each item.
• [Fix] Section header and/or footer should work properly.
• [Fix] Use sectionInset correctly.

Support multiple sections and header/footer for section

• [Add] Multiple sections.
• [Add] Header and/or footer for section.
• [Add] More properties and delegation methods.
• [Change] Remove delegate property, your collectionView's delegate MUST conforms to <CHTCollectionViewDelegateWaterfallLayout> protocol.