Change behaviour if fully qualified classname is passed in as string

If one calls $em->getRepository('\Entity') (leading \ !) then Doctrine's behaviour is quite undefined, as it would result e.g. in a "mapping file '.entity.dcm.yml' not found" error. (leading dot!)

PHP, however, seems to allow the leading \, but Doctrine does not. In fact, the leading \ is not necessary as classnames in strings are always fully qualified.

So Doctrine should either:

1.) Throw an exception stating that using a leading backslash is prohibited
2.) Simply strip it.