Added parameters default values for service config

Symfony 2.2, php 5.4
W/o existense of next parameters cannot use this bundle
  [Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException]                                                       
  The service "rollerworks_password_strength.blacklist.provider.sqlite" has a dependency on a non-existent parameter "rollerworks_p  
  assword_strength.blacklist.sqlite.dsn".

  [Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException]                                                       
  The service "rollerworks_password_strength.blacklist.validator" has a dependency on a non-existent parameter "rollerworks_passwor  
  d_strength.blacklist_provider".
Merge pull request #3 from eliotik/patch-1

fixed exception when not using SQLite blacklist provider.