Allow database assertion to be defined as a bean

[mikebl71]

Problem to solve:
I would like to implement a custom logic for validating actual vs expected datasets.
Spring-dbunit uses DatabaseAssertion objects for such validations.
However, at present, the ExpectedDatabase annotation only accepts DatabaseAssertionMode. But DatabaseAssertionMode is an enum, which means that I cannot extend it in my project.

Proposed solution:
Similar to how you have "dataSetLoader" and "dataSetLoaderBean" elements in DbUnitConfiguration, I've added an "assertionBean" element to ExpectedDatabase. So now I can define any database assertion I like as a bean and refer to it in the annotation.

Example:

@Bean
public DatabaseAssertion dbAssertion() {
    return new CustomDatabaseAssertion();
}

---

@Test
@ExpectedDatabase(value = "classpath:/datasets/Expected.xml",
        assertionBean = "dbAssertion")
public void test() throws Exception {
    ....

---

[Toilal]

It would be great to also define the default assertion bean.

[Toilal]

Thanks for your pull request. I've create another one, more complete because it also allow to defined default assertion bean and/or class.

#120