0.16.0

• [[#112]] Introduce Nullable trait to permit custom Option<T>

• [[#113]] ValueType trait should have a non-panic-ing method

• [[#114]] ValueType revamp

  1. Remove ValueTypeDefault
  2. Change type_name to return String

• [[#115]] Postgres concatenate operator (||)

• [[#117]] Lock support (FOR SHARE, FOR UPDATE) for SELECT statement