Move Adium's dmg_package source, checksum & volumes_dir to attributes file

Notes:

  By default, the recipe will use version 1.5.6 of Adium.
  To use a custom version, put something like this in your
  soloistrc:

    node_attributes:
      sprout:
        adium:
          dmg:
            source: http://sourceforge.net/projects/adium/files/Adium_1.5.7.dmg/download
            checksum: f80408a2c5bc97be8adb35ffeb7c4501392e847edae4e6e24d87da72de7e96b9
            volumes_dir: Adium 1.5.7