Handle py2.7 binary quadratic model bson serialization case
Handle py2.7 binary quadratic model bson serialization case

In python 2.7 bytes is an alias for str which causes issues when
attempting to serialize with bson. We handle this by adding a check
in BQM.to_bson that uses bson.binary.Binary to signal to the
encoder that we wish to store some of the bytes as binary data

oneklc/dimod,oneklc/dimod