Add typespecs for Ecto types

[mcasper]

This allows users to define complete typespecs for their Version modules, without dialyzer (correctly) complaining about

:0:unknown_type
Unknown type: ExAudit.Type.Action.t/0.

:0:unknown_type
Unknown type: ExAudit.Type.Patch.t/0.

:0:unknown_type
Unknown type: ExAudit.Type.Schema.t/0.

[mcasper]

Thank you for the corrections @narrowtux, they've been pushed up!