Add startup message

[operdeck]

library(googleVis) gives startup msg like this, we could do something similar

library(googleVis) Creating a generic function for ‘toJSON’ from package ‘jsonlite’ in package ‘googleVis’

Welcome to googleVis version 0.6.2

Please read Google's Terms of Use before you start using the package: https://developers.google.com/terms/

Note, the plot method of googleVis will by default use the standard browser to display its output.

See the googleVis package vignettes for more details, or visit http://github.com/mages/googleVis.

To suppress this message use: suppressPackageStartupMessages(library(googleVis))