DATAJPA-12 - Added Sort implementations for JPA meta-model API and Querydsl.

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. Enabled static JPA meta-model generation in pom.xml.

Enhanced Querydsl to generate appropriate left joins when sorting by nested (singular) association properties. Converted XML configuration for QueryDslRepositorySupportIntegrationTests into JavaConfig.

Original pull request: #54.
DATAJPA-12 - Design changes in JpaSort.

Decided to go with a simpler way of building up attribute paths on JpaSort to avoid the need to work with JPA Path instances (and thus the EntityManager) entirely. Added shortcut constructors to JpaSort that take a vararg of Attribute or PluralAttribute respectively.

Interestingly, the test cases still have to be integration tests as the  fields in the statically generated meta-model are null until the EntityManagerFactory bootstrap process enhances them to contain actual values. So no real unit tests unfortunately.

Consolidated tests cases for MailMessageRepository into one class, especially to avoid the configuration QueryDslRepositorySupportIntegrationTests to interfere with the newly added tests. Also rather use SampleConfig configuration class to allow the test framework's caching mechanism to kick in.

Original pull request: #54.