Add a command to drop the dbal event store schema.

This is useful when automating end-to-end tests.
Import `\Exception` in the Bundle commands.
configured and ran php-cs-fixer
Merge pull request #79 from lrqdo/drop_event_store_command

Add a command to drop the dbal event store schema.
Merge pull request #84 from othillo/php-cs-fixer

configured and ran php-cs-fixer