Add ability to use own custom cells, provide example.

[Jessicardo]

• Fix compiler/linker error.
  • CoreGraphics Framework missing from test
• Add ability to own custom cells, provide example.
  • Ability to specify custom height for parent and child cells
  • Ability to specify custom parent cell layout & indicator in the custom cell
  • Ability to specify custom child cell layout
  • Example modified to showcase these new features
• Add Single choice expand option