invoke api via tools:rstudio (v0.99 compatibility)

The manipulate package is now available on CRAN and as a result no longer distributed with RStudio. If we can also get rid of the rstudio package then RStudio won't have any packages to install or manage (which will be a big improvement as this has always been a bit messy). In v0.99 I moved the functions previously located in the rstudio package (e.g. preivewRd, viewer, etc.) into the "tools:rstudio" namespace and prefxied them with .rs.api. 

This change attempts to lookup API functions first in tools:rstudio and then as a fallback in the rstudio package. As a result it should work with both older and newer versions of RStudio.