feat: update to JPA 2.1 and relevant hibernate version
feat: update Java version to 8 (mostly for new Date/Time API)
feat: leverage JPA 2.1 features for custom PostgreSQL types (see PGObjectConverter)
feat: initial experimental support of JSR310 (new Date/Time API in Java 8)
feat: experimental support of JSON and LTree types (see LTree/LTreeConverter and JSONConverter)
refactor: replace Specifications anon. classes with lambdas
refactor: comply with jdk 8 javadoc