Merge pull request #22 from kschaef/master

Added AK, HI, cleaned some things
added explorer prelims
added function to launch Dora
diverging colors
Merge pull request #23 from kschaef/Dora

added explorer prelims
Fixed explore_counties() man pages.
In shiny_choro, no need to check args lengths before match.arg().
Simplified imports in shiny_choro. Also, fixed file.copy error.
Cleaned up some docs in shiny_choro.
Some cleanup in the Shiny app.
Improved shiny_choro's initial loading speed. #26
Slight speedup/cleanup in Shiny server.r
Shiny app is slightly faster when filtering. #27
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.