Allow tagging entity listeners

This allows tagging a service with an 'doctrine.orm.entity_listener'
tag. These tagged services will automatically be registered with the
entity listener resolver.

Note: The changes to the XML fixture cannot be parsed due to a bug in
the Symfony DependencyInjection component. A PR that fixes this issue
can be found at https://github.com/symfony/symfony/pull/9350. When that
PR has been merged, all tests in this commit pass.
Updated documentation
Get default EM name from container parameters
Keep BC with older doctrine versions
Use separate entity listener resolvers per em
Swap default namespace
Merge pull request #222 from sandermarechal/tag-listener

Allow tagging entity listeners