Scan condition operators do not match boto3 condition operators

[Quidge]

I'm looking at the source for DynaModel.scan():

* ``IN``: ``Thing.scan(count__in=[11, 12, 13])``

This is confusingly close to but different than the language for the IN condition operator from boto3 docs:

    def is_in(self, value):
        """Creates a condition where the attribute is in the value,

        :type value: list
        :param value: The value that the attribute is in.
        """
        return In(self, value)

(notice the lack of is_)

When using dynamorm, which of these should I be using? SomeModel.scan(attribute__is_in=['opt1', 'opt2']) or SomeModel.scan(attribute__in=['opt1', 'opt2'])?

I've been receiving odd results using both so I'm trying to rule the usage out first.

[Quidge]

Assuming that PR #101 is correct, this issue would be fixed.

Also, lol. @smtheard and I work at the same co. I didn't know that he had this PR open.