Use varchars for string columns

[tiger docs](https://www.census.gov/geo/maps-data/data/pdfs/tiger/tgrshp2013/TGRSHP2013_TechDoc_A.pdf) specify the primary key columns here are strings. It's silly, but that's what the docs say.

Keep the primary key additions from 00ef2750c27662f444c94e20a06b64cc0e82acef
Merge pull request #2 from pnorman/patch-2

Use varchars for string columns