Add support for storing role and meta data as json

Encode empty roles/meta data as NULLs in the database

Favor using Data.Text over ByteStrings for storing JSON text in the
database.

This patch requires the latest sqlite-simple version from github.
This will be released shortly on hackage.
Merge pull request #9 from alexanderkjeldaas/support_roles_and_meta_squash

Add support for storing role and meta data as json