DATAREST-228 - Added Maven Enforcer plugin to ensure project is built with Java 7.

Original pull request: #131.
DATAREST-216 - CrudRepositoryInvoker considers redeclared methods correctly.

CrudRepositoryInvoker now detects re-declared CRUD methods and falls back to reflection invocation if the re-declaration is detected.

Inspired by PR #126 by Nick Weedon but avoiding the introduction of a dependency to Spring Security by using a more precise testing approach.

Original pull request: #126.