2019.05 release

• Added __all__ to modules for better * imports.
• Added cards.
• Added PercentSize, offset, row_break to layout.
• Added carousels.
• Changed the argument list of margin() and padding().
• Added collapses.
• Added dropdowns.
• Added input_groups.
• Added forms.
• Added jumbotrons.
• Added content to the Getting started section of the readme.
• Set up GitHub Pages for the project.
• Bug fixes and documentation fixes.