Update handling of Indicator's negate attribute

Fixes #171 

Update Indicator bindings only to output the negate attribute if it is not None or False. Update the Indicator API to default negate to None, and to allow get/set of negate. Setting the negate value to anything other than 1, '1', or True will cause the object to set negate=None.

This update mirrors how CybOX handles negate (where I saw it, at least.)