1. Made fixes to Intro vignette
2. Fixed bug in p_data when static=TRUE
3. Added unit test: p_extract, p_data, p_funs

Things to think about:

1. p_funs inclusivity see: https://github.com/trinker/pacman/issues/52
2. References needed in Intro Vignette?
3. No way to test p_data when single data set as in issue #51 w/o including a new package like ggvis in dependencies (not that I'm recommending this)

close #51