Added optional dax support

[Will-NOQ]

This is largely based on #470.

Example usage:

class UserModel(Model):
    """
    A DynamoDB User
    """
    class Meta:
        table_name = "dynamodb-user"
        dax_read_endpoints = ['dax://xxxx:8111']
        dax_write_endpoints = ['dax://xxxx:8111']

    email = UnicodeAttribute(null=True)
    first_name = UnicodeAttribute(range_key=True)
    last_name = UnicodeAttribute(hash_key=True)