Decode  dm-serializer output

delayed-job-datamapper produces different serializations than
dm-serialization.  Provide support for parsing both formats in the
decoder in psych_exts.rb.

* The dm-serialization tag includes the DataMapper version before , in the tag

* The dm-serialization encoding is closer to the
  delayed-job-datamapper syck encoding in that it does not have an
  extra attributes hash key containing all the attributes.