Added S3-dispatched input parsers.
Customize dispatch for NULL objects.

Don't provide class for input objects based on `type` field. Provide real NULLs to dispatched functions rather than empty lists.
Revert "Customize dispatch for NULL objects."

This reverts commit f4ada70e564e820c6abbab11c601d7498c5847f3.
Parse incoming NULLs as NA.
Enumerate function variables, setup Roxygen to pass R CMD CHECK, added tests.
Preface shiny input classes with `shiny`. Refine tests to use S3 dispatch.
Merged rstudio/master into branch.
Merge pull request #233 from trestletech/input-registry

Create a user-extensible registry of custom Shiny Input handlers.