v2.0.0

Breaking

• Uses ToProto and ToModel instead of AsModel and AsProto
• timestamppb.Timestamp is encoded as sql.NullTime likewise

Added

• Adds support for wrapped types (google.golang.org/protobuf/types/known/wrapperspb)
  • Implemented originally by @bencurio - Thanks!
  • Adapted here to be nullable, as intended by the types; we map to some type type sql.Null*
  • Adds tests

Upgrading

• Replace AsModel with ToModel
• Replace AsProto with ToProto
• Handle "false nulls" of time.Time{}