Switch to native json types for postgres
Move JSONType to a dedicated fields module
Remove no longer needed validators module
Install sqlalchemy-utils from tarjei's fork

This only temporarly while waiting for
https://github.com/kvesteri/sqlalchemy-utils/pull/58 to be merged and
pushed to PyPI.
Add migration for native postgres JSON type
Clean up internal docs

Remove deleted validators module and add the so far missing fields and
permissions modules.
Merge pull request #23 from streamr/native-postgres-json

Switch to native json types for postgres