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

Brought in the existing handlers for date, matrix, and numeric. Allow users to add/remove handlers via the new addInputHandler and removeInputHandler functions. The addInputHandler function accepts a callback which wil be called with the input value, shinysession object, and element name.
Merge branch 'master' of github.com:rstudio/shiny
Renamed "inputHandler" to "inputTypes" per conversation with Joe.
Merge branch 'master' into input-registry