Saule v1.6

• [BUGFIX] Support belongsTo relationships when data is null (#124 by @goo32)
• [BUGFIX] Use included resource when serialising its links (#130 by @laurence79)
• [BUGFIX] Serialize everything with kebab case by default (#132)
• [BUGFIX] Return reasonable errors for invalid content (#133)
• [FEATURE] Related resources (#137 by @bjornharrtell and @yohanmishkin)
• [FEATURE] Expose properties of resource (#142 by @bjornharrtell)
• [MAINTENANCE] Upgrade dependencies (#143 by @bjornharrtell)
• [BUGFIX] Prevent a resource being included in both the data and included sections (#147 by @laurence79)
• [MAINTENANCE] Move documentation to Github Pages (#148 by @adamalesandro)

Thanks to all contributors!