DATAJPA-12 - Add Sort implementations that use JPA Metamodel API and/or QueryDsl API.

Introduced JpaSort for sorting by JPA meta-model attribute paths.
Introduced JpaMetaModelPathBuilder that can be used to ease the construction of Jpa meta-model attribute paths by the provided static factory method.
Added new testing scenario (MailMessage and MailSender) to avoid to mess up the existing sample classes.
Enhanced Querydsl to generate appropriate left joins when sorting by nested (singular) association properties.
Enabled static JPA meta-model generation in pom.xml.
Converted xml configuration for QueryDslRepositorySupportIntegrationTests into JavaConfig.