the /v1/repositories/<repo_id>/images call now handles the default "library" namespace

Docker 1.2 and earlier did not add the default "library" namespace to this
call. That changed with 1.3, thus we had to adjust the handler.
Merge pull request #22 from pulp/mhrivnak-library-namespace

the /v1/repositories/<repo_id>/images call now handles the default "library" namespace