Don't import entire Shiny namespace

Hi there,

We're days away from submitting Shiny 0.10 to CRAN, and it looks like we have a `validate` function that collides with the rms package. Therefore your package will start warning on R CMD check. According to CRAN policy, we would need you to submit to CRAN an updated version of your package with these changes, before we could submit Shiny 0.10.

It would really help us a lot if you could accept the pull request, update your version number, and resubmit to CRAN. Thanks and sorry for the inconvenience!