add python3 support and fix Rados.mon_command call for ceph pacific

[AzadShM]

these commits:

• add support for python3 Resolves ceph-dash does not work with python3 #66

    * mostly using 2to3
    * added try / except to import urlopen
      to support python2 and python3
    * add try / except to _string_decode_hook
      unicode no longer exists in python3 (the code probably never worked with utf-8 characters
      in python2 anyway but i kept the old behavior, this might not be the best way to deal with it, ideas?)
  

• fix Rados.mon_command() for ceph pacific resolves ceph-dash does not work with ceph pacific 16.2.5 #67

    ceph pacific expects the inbuf argument to be passed as bytes and not str
    this should work fine for older versions too
    see:
    https://bugs.launchpad.net/cinder/+bug/1913449
    https://github.com/ceph/ceph/commit/51a2a72215ac93836ccc37733607b8e97d808e10