Updated counties data set to a tbl_df object.
Updates states data set to a tbl_df object.
Updates zip_codes data set to a tbl_df object.
Updated CSA and CBSA data frames to tbl_df objects.
Updated population_age data frame to tbl_df.
Updated quick_facts to tbl_df object
Updated county_polygons to tbl_df object.
Wrote vignette describing data sets. #28
Added .gitignore file to vignettes folder. #28