Refactor all _layouts and _includes for flexible slidedeck libraries

Restructured to match pattern established in HydeSlides origin repo
so that pattern for layouts and includes is more straight forward for
using ImpressJS or RevealJS
Overhual of theme for slide deck library flexibility

Updated structure and theming pattern for of all styles.
Simplifies modularization of themes so all themes possess
their own dependencies within folder structure.
Change all slide deck layouts to new pattern

Naming pattern changes to hydeslides-revealjs now that support for
impressjs themeing and slide deck layouts in place.
Add ImpressJS as submodule in dependencies/
Update cobalt theme to work with slide deck refactoring
Add new RevealJS slide deck layout implementation
Added Global style for hiding elements (theme agnostic)
Merge pull request #123 from github/hydeslides-update

HydeSlides update with ImpressJS support