Dump png's of ggplot2 and Plotly plots, json's and url's

Visual tests are awesome!
We'll be able to check possible diffs on image files and JSON as we re-run the tests (incl. automatically upon pushing new branches).
/cc @pedrodz @chriddyp 
yeah, i had to move the this file outside of the test folder so that Travis didn't run it. Since I moved it to root, the start of the script had to `cd` into the tests directory