allow deep merging

If two bundles want to enable one listener each by prepending extension
configuration, let them.
@see
https://github.com/stof/StofDoctrineExtensionsBundle/commit/7705f653eb1140dc86c9942b381bda30126e21fe#commitcomment-3944254
Merge pull request #211 from greg0ire/deep_merging

allow deep merging
Fixed the composer requirement

Closes #213
Removed unused private property
Update index.rst
Merge pull request #217 from mgrinko/patch-1

Update index.rst
Merge https://github.com/stof/StofDoctrineExtensionsBundle