Use DefaultPersistenceUnitManager instead of Merging-~

We don't need the MergingPersistenceUnitManager's features so limit the diff
from 1-initial as much as possible.
Use more specific package scan path
Get rid of persistence.xml

Use LocalContainerEntityManagerFactoryBean's packagesToScan-property to specify
the base classpath and no longer define an explicit PersistenceUnitManager.
This allows us to completely remove the persistence.xml file in this scenario.
Get rid of persistence.xml

Uses a small work-around to avoid having to sub-class the
DefaultPersistenceUnitManager: Specify empty packagesToScan so that a default
persistence unit is created, which avoids the need for a persistence.xml.
Remove explicit creation of a persistence unit manager

As long as we don't configure the persistence unit manager in unusual ways, we
can simply use the LocalContainerEntityManagerFactoryBean's default persistence
unit manager.
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Merge branch '1-initial' into 2-jpa-classpathscan
Merge branch '2-jpa-classpathscan' into 3-jpa-custom-classpathscan

Conflicts:
	de.andrena.et14.spring.persistenz/src/main/resources/META-INF/spring-persistence-config.xml
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