Add support for find_and_modify remove kwarg

[angstwad]

Ran into this today. The original document is being returned when using the remove and new kwarg...for example:

...find_and_modify(query=query, remove=True)

It causes the following exception:

  File "/Users/paul4611/python-virtualenvs/quasar/lib/python2.7/site-packages/mongomock/__init__.py", line 349, in find_and_modify
    self.update({'_id':old['_id']}, update)
  File "/Users/paul4611/python-virtualenvs/quasar/lib/python2.7/site-packages/mongomock/__init__.py", line 214, in update
    for k, v in iteritems(document):
  File "/Users/paul4611/python-virtualenvs/quasar/lib/python2.7/site-packages/six.py", line 439, in iteritems
    return iter(getattr(d, _iteritems)(**kw))
AttributeError: 'NoneType' object has no attribute 'iteritems'

[sivel]

Additionally, new and remove cannot coexist. Here is a sample exception from pymongo:

OperationFailure: command SON([('findAndModify', u'test'), ('new', True), ('remove', True), ('query', {'_id': ObjectId('5244a79288b86523defdb136')})]) failed: remove and returnNew can't co-exist

[vmalloc]

Supposed to be fixed now.