Allow users to disable database_cleaner hooks

The hooks that automatically invoke database_cleaner before every
scenario can sometimes cause problems when switching cleaning strategies
between scenarios (issue #215).

To allow a user to take full control of when database_cleaner is
invoked, the configuration option
Cucumber::Rails::Database.autorun_database_cleaner is provided,
defaulting to true.
Merge pull request #232 from urbanautomaton/optional-cleaner-hook

Allow users to disable database_cleaner hooks