DATAJPA-170 - Add support for SpEL in query expressions.

Introduced ExpressionBasedStringQuery to support the SpEL expression template rendering. This allows manually defined queries in either @Query or Spring Data named queries to use SpEL and reference the #entityName. Changed SimpleJpaQuery to use ExpressionBasedStringQuery by default. Added test case for repositories with SpEL expression based query methods.

Original pull request: #25.