Generate-Entities-Console-Command: Adding an 'avoid backup' flag

Introducing a new "avoid creating backup files" flag in the existing command-line options list:

    $ /console orm:generate-entities src/ --nobackup;

Scope: Allowing cli user to avoid the default backup file generation (new `~EntityName.php` file every time),
which is default behavior for now.

= adding only 5 lines to lib/Doctrine/ORM/Tools/Console/Command/GenerateEntitiesCommand.php.