Add support for custom casts

[romanzipp]

Resolves #43

[crazynds]

This branch seems to be giving me some weird warnings:

[WARNING] Could not derive column type from type-name `bpchar` for column `id`
[WARNING] Could not derive column type from type-name `jsonb` for column `payload`
[WARNING] Could not derive column type from type-name `int2` for column `status`
[WARNING] Could not derive column type from type-name `int8` for column `user_id`

I'm using postgresql.