Added support to allow manually declaring a EntityManagerFactoryBean.

Added the necessary @ConditionalOnMissingBean annotations to allow selectively declaring an EntityManagerFactoryBean, a JpaVendorAdapter or a PlatformTransactionManager. Especially the first one might be necessary to make sure the persistence provider evaluates an orm.xml. This unfortunately rules out using the packages to scan feature of Spring's LCEMFB.

I've filed https://jira.springsource.org/browse/SPR-11260 to potentially remove the need for this workaround in Spring itself.