Although README says bson is optional but still being required

[David Lai]

Problem

>>> from montydb import MontyClient
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...\montydb\__init__.py", line 19, in <module>
    from . import utils
  File "...\montydb\utils\__init__.py", line 2, in <module>
    from .io import (
  File "...\montydb\utils\io.py", line 7, in <module>
    from bson import decode_all, BSON
ModuleNotFoundError: No module named 'bson'

These bson import statements should be in somewhere else :

montydb/montydb/utils/io.py

Lines 7 to 14 in ee25354

 from bson import decode_all, BSON 
 from bson.codec_options import CodecOptions 
 from bson.py3compat import string_type 
 from bson.json_util import ( 
     loads as _loads, 
     dumps as _dumps, 
     RELAXED_JSON_OPTIONS as _default_json_opts, 
 )