[GH-603] Add a filter to the import and convert mapping

This issue is created automatically through a Github pull request on behalf of nicolasTheodo:

Url: https://github.com/doctrine/doctrine2/pull/603

Message:

Hi,

I needed to import the mapping of an existing database in order to create doctrine2 entities, but when you run the command, it throw an exception on every table without a primary key.

I tried to use the filter option, but the error still occurs because the exception was throw in the method which get all the metadata informations.

I added a method which get the metadata of some tables according to a filter.

What are your thought bout my issue?