Render cells in drawRect

For performance reasons we now render cells in drawRect
instead of using contraints
Customize cell appearance in collection view

Passing in cell customization as properties prevents
the cells from calling UIAppearance each time they render,
which can be quite expensive
Disable cells with date before current date
Customize cell font with UIAppearance
Bounce cell when selecting
Customizable cell height
Make overlayView optional
Make scroll indicator optional
Customizable header
Update example app
Merge pull request #1 from rechsteiner/updates

Updates