make the configuration dumpable

Symfony does not know how to build the Configuration because it has a
Constructor. Tell it how to do it by implementing getConfiguration().
This makes the dump-reference work with this bundle.
Changed default context

When I create a children block the link with the type of block is not made.
sonata_page_bundle as default_contexts correct the issue.
Merge pull request #156 from Sinjonathan/patch-1

Changed default context
Merge remote-tracking branch 'sonata-project/master'
Fixed response ttl if no block has been rendered
Merge pull request #157 from mremi/fix-ttl

Fixed response ttl if no block has been rendered
Merge pull request #151 from greg0ire/make_the_configuration_dumpable

make the configuration dumpable
Update ContainerBlockService.php

Too many admins registered.
Merge pull request #163 from Warxcell/patch-1

Update ContainerBlockService.php
Merge remote-tracking branch 'sonata-project/master'