Support APT pinning by codename

Following simple patch allows pinning by codename.

man apt_preferences:
       the Codename: line
           names the codename to which all the packages in the directory tree
           belong. For example, the line "Codename: wheezy" specifies that all
           of the packages in the directory tree below the parent of the
           Release file belong to a version named wheezy. Specifying this
           value in the APT preferences file would require the line:

               Pin: release n=wheezy